Philosophical skepticism

Philosophical skepticism (alternatively spelled scepticism in the UK, derived from the Greek σκέψις, skepsis, meaning "inquiry") encompasses a range of philosophical perspectives that fundamentally challenge the attainability of knowledge. This form of skepticism distinguishes itself from other skeptical approaches by extending its rejection even to highly plausible knowledge assertions typically considered basic common sense. Proponents of philosophical skepticism are generally categorized into two main groups: those who deny the very possibility of knowledge, and those who advocate for the suspension of judgment, citing insufficient evidence. This categorization draws parallels with the historical divergence between Academic skeptics and Pyrrhonian skeptics in ancient Greek philosophy. Pyrrhonian skepticism, specifically, is characterized as a practice of withholding judgment, thereby conceptualizing skepticism as a lifestyle conducive to achieving inner tranquility. While some manifestations of philosophical skepticism repudiate all forms of knowledge, others restrict this rejection to particular domains, such as moral doctrines or the nature of the external world. Critics often contend that philosophical skepticism is inherently self-refuting, given that its adherents appear to know that knowledge is unattainable. Further objections frequently highlight its perceived implausibility and detachment from everyday human experience.

Philosophical skepticism (UK spelling: scepticism; from Greek σκέψις skepsis, "inquiry") is a family of philosophical views that question the possibility of knowledge. It differs from other forms of skepticism in that it even rejects very plausible knowledge claims that belong to basic common sense. Philosophical skeptics are often classified into two general categories: Those who deny all possibility of knowledge, and those who advocate for the suspension of judgment due to the inadequacy of evidence. This distinction is modeled after the differences between the Academic skeptics and the Pyrrhonian skeptics in ancient Greek philosophy. Pyrrhonian skepticism is a practice of suspending judgment, and skepticism in this sense is understood as a way of life that helps the practitioner achieve inner peace. Some types of philosophical skepticism reject all forms of knowledge while others limit this rejection to certain fields, for example, knowledge about moral doctrines or about the external world. Some theorists criticize philosophical skepticism based on the claim that it is a self-refuting idea since its proponents seem to claim to know that there is no knowledge. Other objections focus on its implausibility and distance from regular life.

Overview

Philosophical skepticism represents a questioning stance regarding widely accepted knowledge assertions. More broadly, skepticism involves a critical disposition toward various types of knowledge claims. In this general sense, skepticism is prevalent in daily life; for instance, many individuals exhibit ordinary skepticism concerning fields such as parapsychology or astrology, questioning the assertions made by their advocates. However, these same individuals typically do not extend their skepticism to other knowledge claims, such as those presented in standard educational texts. Philosophical skepticism diverges from this ordinary skepticism by challenging even those knowledge claims that are considered fundamental common sense and appear undeniably certain. Consequently, it is occasionally termed radical doubt. In extreme instances, it may even assert that one cannot know propositions like "I have two hands" or "the sun will rise tomorrow." Thus, philosophical skepticism is not a viewpoint commonly embraced by individuals in their daily lives. This rejection of knowledge is typically accompanied by the imperative to suspend judgment regarding the disputed proposition. This implies maintaining an open mind, neither affirming nor denying the proposition, without committing to either stance. Philosophical skepticism frequently stems from the premise that regardless of the certainty one feels about a particular belief, the possibility of error always remains. Based on this observation, it is contended that such a belief does not constitute knowledge. Philosophical skepticism arises from the consideration that this potential for error may apply to most, if not all, beliefs. Given its extensive implications, philosophical skepticism holds significant importance for theories of knowledge, as it challenges their fundamental premises.

Certain definitions characterize philosophical skepticism not merely as the repudiation of some widely accepted knowledge forms, but rather as a comprehensive rejection of all knowledge. From this perspective, while individuals may hold relatively firm beliefs in certain situations, these beliefs are never considered to constitute genuine knowledge. Conversely, less radical manifestations of philosophical skepticism confine this rejection to particular domains, such as the existence of the external world or moral principles. In some instances, knowledge itself is not entirely dismissed, but the possibility of attaining absolute certainty is nevertheless denied.

Few proponents advocate for philosophical skepticism in its most rigorous form; more frequently, it serves as a theoretical instrument for evaluating other theories. This perspective frames it as a philosophical methodology employed to identify vulnerabilities within a theory, either to discredit it or to refine it into a superior iteration. Nevertheless, certain theorists differentiate philosophical skepticism from methodological skepticism: the former challenges the attainability of certainty in knowledge, while the latter systematically scrutinizes all knowledge assertions to discern truth from falsehood. Analogously, scientific skepticism diverges from philosophical skepticism; it represents an epistemological stance where the validity of claims unsupported by empirical evidence is questioned. Practically, the term typically refers to the critical assessment of claims and theories perceived as pseudoscience, rather than the standard discourse and challenges inherent in scientific inquiry.

In ancient philosophy, skepticism was conceived not merely as a theory concerning the nature of knowledge, but as a comprehensive way of life. This perspective is predicated on the belief that suspending judgment across various matters fosters inner tranquility, thus contributing to the skeptic's eudaimonia.

Classification

Skepticism can be categorized based on its scope. Local skepticism pertains to doubt regarding specific domains of knowledge (e.g., moral skepticism, skepticism concerning the external world, or skepticism about other minds), while radical skepticism asserts the impossibility of knowing anything at all, even the fact of not knowing.

Skepticism may also be classified by its methodology. Western philosophy identifies two primary approaches to skepticism. Cartesian skepticism, somewhat inaccurately named after René Descartes (who, despite employing traditional skeptical arguments in his Meditations to underpin his rationalist epistemology, was not a skeptic himself), endeavors to demonstrate the dubitability of any asserted knowledge claim. Agrippan skepticism, conversely, prioritizes the concept of justification over the mere possibility of doubt. From this perspective, no method of justifying a claim proves sufficient: justifying a claim by appealing to other claims results in an infinite regress; a dogmatic assertion does not constitute a valid justification; and circular reasoning fails to substantiate its conclusion.

Skeptical Scenarios

A skeptical scenario constitutes a hypothetical situation employed in arguments to foster skepticism regarding a specific claim or category of claims. Typically, such a scenario posits a deceptive entity capable of misleading our senses and invalidating the justification of knowledge ordinarily considered sound. Its purpose is to challenge our everyday knowledge assertions, given that we cannot definitively rule out the veracity of these skeptical scenarios. These scenarios have garnered significant scholarly interest within modern Western philosophy.

The inaugural prominent skeptical scenario in modern Western philosophy is presented in René Descartes's Meditations on First Philosophy. Concluding the First Meditation, Descartes posits: "I will suppose ... that some evil demon of the utmost power and cunning has employed all his energies to deceive me."

• The "evil demon problem," alternatively termed "Descartes's evil demon," was initially conceptualized by René Descartes. This thought experiment posits the existence of a malevolent entity capable of systematically deceiving an individual into accepting as true everything they believe.
• The "brain in a vat" hypothesis is articulated using contemporary scientific terminology. It postulates the possibility of an individual existing as a disembodied brain, sustained in a vat and receiving fabricated sensory input from a malevolent scientist. Furthermore, it contends that because a brain in a vat would lack any means of discerning its condition, one cannot definitively demonstrate that one is not, in fact, a brain in a vat.
• The "dream argument," advanced by both René Descartes and Zhuangzi, posits that reality may be indistinguishable from a dream state.
• The "five-minute hypothesis," prominently introduced by Bertrand Russell, proposes that it is impossible to conclusively demonstrate that the universe was not created merely five minutes ago, complete with fabricated memories and spurious evidence indicating a longer existence.
• The "simulated reality hypothesis," also known as the "Matrix hypothesis," posits that all individuals, or potentially the entire cosmos, could exist within a computer simulation or a virtual reality environment.
• The solipsistic theory asserts that an individual's knowledge of the external world is merely an illusion generated by the Self.

Epistemological Skepticism

As an epistemological perspective, skepticism fundamentally questions the very possibility of knowledge. This differs from other forms of skeptical inquiry, such as Cartesian skepticism, by challenging the existence of knowledge universally rather than focusing on specific categories of knowledge.

Skeptics contend that mere belief in a proposition does not warrant a claim of knowledge regarding it. Consequently, skeptics stand in opposition to foundationalism, a philosophical stance asserting the existence of fundamental beliefs that are either self-evident or require no external justification. (An illustrative instance of such foundationalism is presented in Spinoza's Ethics.)

Utilizing arguments such as the Münchhausen trilemma and the problem of the criterion, skeptics assert that the attainment of any certain belief is impossible. This stance is commonly referred to as "global skepticism" or "radical skepticism." Conversely, foundationalists have invoked the same trilemma to justify the inherent validity of basic beliefs. Epistemological nihilism, distinct from general skepticism, specifically denies the possibility of human knowledge, though not necessarily knowledge in its broadest sense.

Epistemological skepticism is broadly categorized into two distinct forms: mitigated and unmitigated skepticism. While contrasting, both represent genuine expressions of skeptical thought. Mitigated skepticism disallows "strong" or "strict" knowledge claims but permits certain weaker assertions, which may be termed "virtual knowledge," provided they are based on justified belief. Some proponents of mitigated skepticism are also fallibilists, contending that knowledge does not necessitate absolute certainty. These mitigated skeptics maintain that knowledge does not require certainty and that numerous beliefs are, in practical terms, sufficiently certain to guide actions for leading significant and meaningful lives. In contrast, unmitigated skepticism repudiates both virtual and strong knowledge claims. The categorization of knowledge as strong, weak, virtual, or genuine can vary significantly based on an individual's perspective and their definition of knowledge. Unmitigated skeptics posit that objective truths are inherently unknowable and advocate for an isolated existence to achieve mental tranquility, reasoning that all phenomena are in flux and relative. Consequently, the refusal to render judgments is paramount, as only probable opinions, not definitive knowledge, are attainable.

Critique

Philosophical skepticism has attracted diverse forms of criticism. Some critiques characterize it as inherently self-refuting, while others argue it is implausible, psychologically untenable, or merely an unproductive intellectual exercise. This perspective stems from the observation that philosophical skepticism, while rejecting the possibility of knowledge, simultaneously appears to advance its own knowledge claims. For instance, the assertion that "no knowledge exists" itself constitutes a knowledge claim. This paradox is particularly pertinent for forms of philosophical skepticism that deny all types of knowledge. A global skeptic, for example, denies the rational justification of any claim yet proceeds to offer arguments intended to rationally justify this very denial. In response to this objection, some philosophical skeptics have limited their denial of knowledge to specific domains, thereby not rejecting knowledge universally. Another defense frames philosophical skepticism not as a theoretical framework but as a methodological tool. In this capacity, it can be effectively employed to challenge and refine philosophical systems, notwithstanding its limitations as a standalone theory.

A prominent critique asserts that philosophical skepticism is profoundly counterintuitive, given its significant divergence from ordinary human experience. For instance, simultaneously suspending all beliefs appears highly impractical, if not psychologically unattainable. Furthermore, even if such a state were achievable, it would be ill-advised, as "the complete skeptic would wind up starving to death or walking into walls or out of windows." While acknowledging that some arguments may support philosophical skepticism, this critique posits that these arguments are insufficient to substantiate such a radical conclusion. Proponents of common-sense philosophy align with this perspective, contending that ordinary, intuitive beliefs possess greater reliability than the intricate arguments advanced by skeptics. George Edward Moore, for instance, sought to counter skepticism regarding the external world's existence not through engagement with its complex arguments, but by presenting a straightforward observation: the presence of his two hands. Moore considered this observation a dependable source of knowledge, inherently incompatible with external world skepticism, as it necessarily implies the existence of at least two physical objects.

A related critique characterizes philosophical skepticism as an "idle academic exercise" or a "waste of time." This viewpoint frequently stems from the premise that, owing to its initial implausibility and detachment from daily existence, it offers minimal or no practical utility. Arthur Schopenhauer, in this context, likened radical skepticism to an impregnable border fortress whose garrison poses no threat because it never ventures beyond its walls, suggesting it is best disregarded. Conversely, a defense of philosophical skepticism highlights its significant influence on the broader history of philosophy, extending beyond the domain of skeptical thinkers. This impact is attributed to its inherent critical stance, which consistently challenges the epistemic underpinnings of diverse philosophical theories. Such challenges have frequently spurred creative responses from other philosophers, prompting modifications to affected theories to circumvent skeptical objections.

Pierre Le Morvan identifies two prevalent negative reactions to philosophical skepticism. The first perceives it as a menace to all philosophical theories and endeavors to refute it. The second perspective considers philosophical skepticism a futile diversion that ought to be entirely circumvented. Le Morvan, however, advocates for a constructive third approach: employing skepticism as a philosophical instrument in specific instances to transcend biases and cultivate practical wisdom.

The Historical Trajectory of Western Skepticism

Skepticism in Ancient Greece

Ancient Greek skeptics did not embody the contemporary understanding of "skeptics," which implies selective or localized doubt. Their primary concerns were epistemological, observing the inadequacy of support for truth claims, and psychotherapeutic, recognizing that beliefs often induced mental disquiet.

The systematic tradition of Western skepticism can be traced back at least to Pyrrho of Elis (born c. 360 BCE) and arguably even to Xenophanes (born c. 570 BCE). Elements of skepticism are also evident among the "5th century sophists [who] developed forms of debate that served as precursors to skeptical argumentation, priding themselves on persuasively arguing for both sides of an issue."

Within Hellenistic philosophy, Pyrrhonism and Academic Skepticism constituted the two principal schools of skeptical thought. Consequently, the terms Academic and Pyrrhonist frequently became synonymous with skeptic.

The Philosophy of Pyrrhonism

Consistent with other Hellenistic philosophies, Pyrrhonism aimed for eudaimonia, which its adherents pursued by attaining ataraxia (a state of mental tranquility). This tranquility, they discovered, could be fostered by cultivating epoché (suspension of judgment) concerning matters that were not self-evident. Epoché was achievable by juxtaposing opposing dogmas to erode conviction and by rigorously scrutinizing the justification of any given belief. To bolster this investigative approach, Pyrrhonists formulated the aforementioned skeptical arguments (the Ten Modes of Aenesidemus and the Five Modes of Agrippa), which served to illustrate the unjustifiable nature of beliefs:

Pyrrho of Elis: Biographical Context

As documented in an account of Pyrrho's life by his student Timon of Phlius, Pyrrho advocated a specific path to achieving happiness and tranquility:

The phenomena themselves are equally indifferent, unstable, and indeterminate; consequently, neither our sensory perceptions nor our judgments can be definitively categorized as true or false. Therefore, reliance on these faculties is unwarranted. Instead, one should maintain a state of suspended judgment, free from preconceived notions, bias, or vacillation. This entails asserting of every phenomenon that its existence is no more certain than its non-existence, or that it both exists and does not exist, or that it neither exists nor does not exist.

Aenesidemus

Pyrrhonism declined as a philosophical movement after the demise of Pyrrho's disciple, Timon. The Academy gradually adopted a more dogmatic stance, leading Aenesidemus, in the first century BCE, to criticize the Academics as "Stoics contending against Stoics." He subsequently separated from the Academy to re-establish Pyrrhonism. Aenesidemus's most notable contribution to skeptical thought was his lost treatise, Pyrrhonian Discourses, whose content is primarily accessible through the writings of Photius, Sextus Empiricus, and, to a lesser degree, Diogenes Laërtius. The skeptical arguments predominantly attributed to Aenesidemus are the aforementioned ten modes, formulated to facilitate epoche.

Sextus Empiricus

The writings of Sextus Empiricus (circa 200 CE) constitute the primary extant record of ancient Pyrrhonism. Significantly prior to Sextus's era, the Academy had relinquished its skeptical stance and ceased to exist as a formal institution. Sextus systematically compiled and elaborated upon the Pyrrhonist skeptical arguments, predominantly targeting the Stoics, but also encompassing critiques of all Hellenistic philosophical schools, including the Academic skeptics.

Sextus, recognized as the most systematic surviving author among Hellenistic skeptics, identified at least ten modes of skepticism. These modes can be categorized into three principal areas of doubt: concerning the subjective perceiver, the objective world, and the relationship between the perceiver and the world. The subsequent arguments elucidate these modes.

From a subjective perspective, the capacities of sensory perception and reasoning exhibit variability across individuals. Given that knowledge is derived from either of these faculties, and neither is consistently reliable, the very foundation of knowledge appears problematic. For example, an individual with color blindness perceives the world distinctly from those with typical vision. Furthermore, it is not justifiable to prioritize reason as a basis for knowledge, meaning that one cannot assert that rational animals possess superior knowledge compared to irrational animals, as the latter demonstrate proficiency in navigating their surroundings, thereby implying a form of 'knowing' regarding certain environmental aspects.

Secondly, an individual's personality may also exert influence over their observations, as it is contended that preferences originate from sensory impressions, and thus variations in preferences can be ascribed to divergent ways in which individuals are affected by an object. (Empiricus, p. 56)

Third, the perceptions derived from each individual sense appear to lack commonality with those from other senses; for instance, the visual perception of 'red' bears minimal relation to the tactile sensation of touching a red object. This divergence becomes evident when our senses produce conflicting information: a mirage, for example, exhibits specific visible characteristics yet elicits no response from other sensory modalities. Consequently, other senses invalidate the visual impressions. Furthermore, an individual might possess insufficient sensory capacities to comprehend the world comprehensively; the acquisition of an additional sense could potentially reveal aspects of reality that the existing five senses cannot convey. Therefore, if our senses can be demonstrated as unreliable through cross-sensory comparison, and if they are potentially incomplete (lacking a hypothetical 'more perfect' sense), it logically follows that all our sensory faculties might be untrustworthy. (Empiricus, p. 58)

Fourth, the conditions under which perception occurs can be categorized as either natural or unnatural, such as states of wakefulness or sleep, respectively. However, it remains entirely plausible that phenomena in the world genuinely correspond to their appearances during unnatural states (e.g., if reality were an elaborate dream). (Empiricus, p. 59)

Reasons for doubt can stem from the relationship between objective "facts" and subjective experience. The spatial attributes of objects, such as their positions, distances, and locations, appear to influence how an individual perceives them. For instance, a portico might seem tapered when viewed from one end but symmetrical from another, presenting distinct characteristics. Since these are different features, asserting that an object simultaneously possesses both properties implies a belief in contradictory attributes. Given the absurdity of this, one must suspend judgment regarding an object's actual properties when confronted with conflicting experiences. (Empiricus: 63)

Furthermore, it can be observed that perceptions are, in a sense, influenced by experience. Any given perception—for example, of a chair—is invariably contextualized (e.g., next to a table, on a mat). Consequently, understanding is often limited to how ideas manifest within their associated contexts, precluding knowledge of an object's true nature, revealing only its appearance within a specific setting. (Empiricus: 64)

Following this line of reasoning, a skeptic might assert the relativity of all things by arguing that:

1. Absolute appearances either diverge from relative appearances or they do not.
2. If absolute appearances do not differ from relative ones, then they are inherently relative.
3. However, if absolute appearances do differ from relative ones, they are still relative, because all distinctions imply a relation to that from which they differ; thus, to "differ" from something is to be relative to it. (Empiricus: 67)

Finally, grounds for disbelieving in certain knowledge arise from the challenges in comprehending objects in isolation. Items observed individually may present characteristics significantly different from when they are seen in large quantities; for example, the shavings of a goat's horn are white when separated, yet the intact horn appears black.

Skeptical Arguments

The ancient Greek Pyrrhonists developed various arguments to demonstrate that claims about reality cannot be adequately substantiated. Two prominent sets of these arguments are widely recognized. The older collection is known as the ten tropes of Aenesidemus, though it remains uncertain whether he originated these tropes or merely systematized them from earlier Pyrrhonist works. These tropes provide rationales for epoché, or the suspension of judgment, and include the following:

1. Different animal species exhibit distinct modes of perception;
2. Comparable variations are observed among individual human beings;
3. For a single individual, sensory information can be inherently contradictory;
4. Moreover, perceptions fluctuate over time due to physical changes;
5. Additionally, this data varies according to local relationships;
6. Objects are apprehended only indirectly, mediated by substances such as air or moisture;
7. These objects are in a state of perpetual transformation regarding color, temperature, size, and motion;
8. All perceptions are relational and mutually influential;
9. Repeated exposure and custom diminish the critical assessment of our impressions;
10. All individuals are raised with diverse beliefs, under different legal systems and social conditions.

Another set of arguments is known as the five tropes of Agrippa:

1. Dissent – The uncertainty evidenced by the divergence of opinions among philosophers and the general populace.
2. Progress ad infinitum – The principle that all proof relies on matters that themselves require proof, leading to an unending sequence, also known as the regress argument.
3. Relation – All things undergo change as their relationships are altered, or when viewed from different perspectives.
4. Assumption – The asserted truth is founded upon an unsupported premise.
5. Circularity – The validation of the asserted truth involves a circularity of proofs.

According to Victor Brochard, "the five tropes can be regarded as the most radical and most precise formulation of philosophical skepticism that has ever been given. In a sense, they are still irresistible today."

Academic Skepticism

Pyrrho's philosophical contributions subsequently influenced the Platonic Academy, manifesting initially as Academic skepticism within the Middle Academy, led by Arcesilaus (c. 315–241 BCE), and subsequently in the New Academy, under Carneades (c. 213–129 BCE). Clitomachus, a disciple of Carneades, construed his mentor's philosophical framework as proposing an epistemological model grounded in verisimilitude. The Roman statesman and philosopher, Cicero, also embraced the skepticism characteristic of the New Academy, despite the concurrent emergence of a more dogmatic inclination within the school.

Augustine's Engagement with Skepticism

In 386 CE, Augustine released Contra Academicos (Against the Academic Skeptics), a work that contested the assertions of the Academic Skeptics (266–90 BCE) based on the subsequent arguments:

• The Objection from Error: Augustine contended, through logical reasoning, that philosophical skepticism does not culminate in happiness, contrary to the assertions of the Academic Skeptics. His argument is summarized as follows:
  1. A wise individual lives in accordance with reason, thereby achieving happiness.
  2. An individual who perpetually seeks knowledge without attaining it remains in a state of error.
  3. The Imperfection Objection: Individuals in error cannot achieve happiness, as error constitutes an imperfection, and happiness is incompatible with such a state.
  4. Conclusion: Consequently, an individual who continues to pursue knowledge cannot attain happiness.
• The Error of Non-Assent: Augustine posited that the suspension of belief does not entirely preclude an individual from committing error. His argument is outlined below.
  1. Introduction of the error: Assuming P is true, if an individual refrains from believing P by suspending judgment to avert error, that individual simultaneously commits an error.
  2. The Anecdote of the Two Travelers: Two travelers, A and B, endeavor to reach a common destination. Upon encountering a fork in the road, a humble shepherd advises them to proceed left. Traveler A promptly accepts this counsel and successfully arrives at the intended location. Conversely, Traveler B suspends judgment, opting instead to follow the guidance of a well-attired townsman who suggests going right, finding his advice more compelling. Nevertheless, the townsman is, in fact, a samardocus (con man), resulting in Traveler B's failure to reach the correct destination.
  3. The Anecdote of the Adulterer: An individual suspends the belief that adultery is morally wrong and proceeds to commit adultery with another man's wife, swayed by its perceived persuasiveness. According to Academic Skepticism, this individual could not be held accountable, as his actions were based on what he found persuasive, without assenting to a definitive belief.
  4. Conclusion: Therefore, the suspension of belief renders individuals vulnerable to error, as conceptualized by the Academic Skeptics.
  5. The Resurgence of Skepticism in the Sixteenth Century

Skepticism's revival in the sixteenth century

Francisco Sanches's treatise, That Nothing is Known (published in 1581 as Quod nihil scitur), stands as a pivotal work within Renaissance skepticism.

Michel de Montaigne (1533–1592)

Michel de Montaigne, the preeminent figure in the sixteenth-century revival of Skepticism, documented his investigations into Academic Skepticism and Pyrrhonism within his Essais.

His most significant contributions to skeptical thought are found in an essay primarily composed between 1575 and 1576, titled "Apologie de Raimond Sebond." During this period, Montaigne was engaged in reading Sextus Empiricus and endeavoring to translate Raimond Sebond's works, which included a demonstration of Christianity's natural existence. The scholarly reception of Montaigne's translations encompassed critiques of Sebond's proof. Montaigne addressed some of these criticisms in the Apologie, offering a defense of Sebond's logic that exhibited a skeptical character akin to Pyrrhonism. His counter-argument proceeded as follows:

1. Critics who asserted the weakness of Sebond's arguments inadvertently revealed the egoistic human tendency to presume the superiority of one's own logic over that of others.
2. Numerous animal species demonstrate superiority to humans in various aspects. To substantiate this assertion, Montaigne cited instances of dogs exhibiting logical reasoning and constructing their own syllogisms to comprehend their environment, an example previously employed by Sextus Empiricus.
3. Given that animals also possess rationality, the excessive veneration of human mental capacities constitutes a fallacy—a manifestation of human folly. Consequently, one individual's reason cannot be definitively deemed superior to another's.
4. Furthermore, religious doctrines sometimes advocate for ignorance, positing that individuals can attain faith by dutifully adhering to divine instructions for learning, rather than relying on personal logic.

Marin Mersenne (1588–1648)

Marin Mersenne, a polymath encompassing roles as an author, mathematician, scientist, and philosopher, initially championed science and Christianity against atheists and Pyrrhonists. Subsequently, he dedicated himself to fostering the advancement of science and the "new philosophy," a movement that included figures such as Gassendi, Descartes, Galileo, and Hobbes. His significant contribution concerning Skepticism is La Verité des Sciences, where he posits that despite the inherent limitations in discerning the true essence of phenomena, scientific inquiry enables the formulation of definitive laws and principles governing sense-perceptions.

Furthermore, Mersenne contended that universal doubt is unwarranted, citing several reasons:

• There exists a consensus among humans regarding certain fundamental observations, such as the relative size difference between an ant and an elephant.
• Natural laws, exemplified by optics, regulate sensory perceptions, thereby facilitating the correction of inaccuracies.
• Humanity has developed instruments, including rulers and scales, to precisely measure objects and resolve perceptual ambiguities, such as the apparent bending of oars in water, the iridescent appearance of pigeons' necks, or the visual distortion of distant round towers.

A Pyrrhonist might counter these assertions by arguing that sensory deception leads to an infinite regress or circularity in knowledge acquisition. Mersenne, however, refuted this, maintaining that widely accepted empirical principles can be hypothesized and rigorously tested over time to confirm their enduring validity.

Moreover, Mersenne posited that if universal doubt were possible, then doubt itself could be doubted ad infinitum, implying that some truth must ultimately exist. He further highlighted the extensive body of mathematical, physical, and other scientific knowledge validated through repeated experimentation and possessing practical utility. Significantly, Mersenne was among the limited number of philosophers who embraced Hobbes's groundbreaking ideology, perceiving it as an emergent science of humanity.

Skepticism in the Seventeenth Century

Thomas Hobbes (1588–1679)

During his extended residence in Paris, Thomas Hobbes participated actively within a prominent intellectual circle that included leading skeptics such as Gassendi and Mersenne, whose work centered on skepticism and epistemology. In contrast to his skeptical associates, Hobbes did not primarily address skepticism as a central theme in his own writings. Nevertheless, his contemporaries identified him as a religious skeptic due to his questioning of the Mosaic authorship of the Pentateuch and his political and psychological interpretations of religious phenomena. While Hobbes himself refrained from challenging other religious tenets, his reservations regarding Mosaic authorship significantly impacted religious traditions and prepared the ground for subsequent religious skeptics, including Spinoza and Isaac La Peyrère, to further scrutinize foundational beliefs within the Judeo-Christian religious framework. Hobbes's response to skepticism and epistemology was notably political: he posited that both moral and religious knowledge were inherently relative, lacking any absolute standard of truth. Consequently, he argued that specific truth standards concerning religions and ethics were formulated and institutionalized for political expediency, aiming to establish a functional government and a stable society.

Baruch Spinoza and Religious Skepticism

Baruch Spinoza stands as a pioneering figure among European philosophers who embraced religious skepticism. Profoundly conversant with Cartesian philosophy, he innovatively applied the Cartesian method to religious discourse, employing it for the analysis of sacred texts. Spinoza aimed to challenge the epistemological assertions of the Judeo-Christian-Islamic religious framework by scrutinizing its dual foundations: Scripture and Miracles. He posited that all Cartesian, or rational, knowledge ought to be universally accessible. Consequently, he argued that the Scriptures, with the exception of those attributed to Jesus, should not be regarded as divine, esoteric knowledge but rather as products of prophetic imagination. This assertion effectively diminished the Scriptures' role as a basis for knowledge, reclassifying them as mere ancient historical documents. Furthermore, Spinoza dismissed the possibility of miracles, contending that their perceived miraculous nature stemmed solely from human ignorance regarding natural phenomena. Through his rejection of the validity of both Scriptures and Miracles, Spinoza dismantled the bedrock of religious knowledge claims, thereby establishing his conception of Cartesian knowledge as the singular arbiter of truth. While profoundly skeptical of religious doctrines, Spinoza exhibited an intense anti-skepticism toward reason and rationality. He firmly affirmed the legitimacy of reason by linking it to the recognition of God, suggesting that skepticism concerning the rational pursuit of knowledge arose not from inherent flaws in rational knowledge itself, but from a fundamental incomprehension of God. Thus, Spinoza's unique blend of religious skepticism and rational anti-skepticism significantly reshaped epistemology by delineating theological from rational knowledge claims.

Pierre Bayle (1647–1706)

Pierre Bayle, a late 17th-century French philosopher, was characterized by Richard Popkin as a "supersceptic" for his radical extension of the skeptical tradition. Born into a Calvinist family in Carla-Bayle, Bayle initially converted to Catholicism before reverting to Calvinism. This religious oscillation prompted his relocation from France to the more religiously tolerant Holland, where he resided and pursued his work for the remainder of his life.

Bayle contended that truth is unattainable through reason, asserting that all human attempts to acquire absolute knowledge are destined to fail. His primary methodology was profoundly skeptical and deconstructive: he meticulously scrutinized and analyzed existing theories across all domains of human knowledge to expose their inherent logical deficiencies and, consequently, their fundamental absurdity. In his magnum opus, Dictionnaire Historique et Critique (Historical and Critical Dictionary), Bayle assiduously pinpointed logical inconsistencies in numerous historical works, thereby underscoring the ultimate futility of rationality. This comprehensive rejection of reason ultimately led him to conclude that faith represents the sole and ultimate path to truth.

The true intent underlying Bayle's profoundly deconstructive works remains a subject of scholarly debate. While some scholars have categorized him as a Fideist, others have posited that he was a clandestine atheist. Regardless of his personal convictions, Bayle exerted considerable influence on the nascent Age of Enlightenment through his dismantling of foundational theological concepts and his advocacy for religious tolerance and atheism within his writings.

Skepticism in the Age of Enlightenment

David Hume (1711–1776)

David Hume emerged as one of the most influential proponents of philosophical skepticism during the Age of Enlightenment, and a prominent figure within both the Scottish Enlightenment and British Empiricism. He particularly advocated skepticism concerning inductive reasoning and critically examined the foundations of morality, thereby formulating the renowned is–ought problem. His skeptical methodology is widely regarded as even more radical than that proposed by Descartes.

Hume posited that any intelligible concept must originate either as a direct mental replication of a sensory impression or as a novel synthesis of multiple impressions. Consequently, he contended that human endeavors such as religion, superstition, and metaphysics, lacking foundations in actual sense-impressions, possess logically indefensible claims to knowledge. Moreover, Hume illustrated that science itself constitutes a psychological phenomenon, rooted in the association of ideas, particularly the assumption of cause-and-effect relationships, which similarly lacks grounding in sensory experience. This perspective led him to conclude that even scientific knowledge is logically unwarranted, being neither objective nor verifiable, but rather a tenuous conjecture derived from the mind's perception of consistent correlations between discrete occurrences. Hume's analysis thus culminates in profound skepticism concerning the attainability of certain knowledge, ultimately suggesting that, at its most robust, a science of human nature serves as the "only solid foundation for the other sciences."

Immanuel Kant (1724–1804)

Immanuel Kant (1724–1804) endeavored to establish a foundation for empirical science, countering David Hume's skeptical critique of causality. Hume (1711–1776) had contended that no analysis of cause and effect could be reconciled with the empiricist framework primarily articulated by John Locke (1632–1704). However, Kant's effort to ground empirical scientific knowledge simultaneously restricted the scope of other forms of knowledge, particularly what he termed "metaphysical knowledge." Consequently, Kant deemed empirical science legitimate, while largely dismissing metaphysics and philosophy as illegitimate. The primary exception to this distinction between legitimate and illegitimate knowledge was ethics, whose principles Kant asserted could be apprehended through pure reason, independent of the empirical knowledge required for scientific inquiry. Therefore, regarding metaphysics and philosophy broadly (with ethics as the sole exception), Kant adopted a skeptical stance. This skepticism, alongside G. E. Schulze's explicit skepticism, stimulated significant discourse on skepticism within German idealist philosophy, notably influencing Hegel. Kant's central tenet was that the true nature of reality (the noumenon or thing-in-itself) remained beyond the grasp of human reason, even though the empirical world of nature was accessible to human understanding; thus, ultimate reality could never be known. In opposition to Kant, Hegel argued that while Kant correctly identified that "finite" concepts of "the understanding" (as Hegel termed them) precluded knowledge of reality, humanity was not limited to such concepts and could indeed attain knowledge of reality through "infinite concepts" emerging from self-consciousness.

Skepticism in Twentieth-Century and Contemporary Philosophy

G. E. Moore notably advanced the "Here is one hand" argument to counter skepticism in his 1925 essay, "A Defence of Common Sense." Moore asserted that he could demonstrate the existence of the external world by simply articulating the following while displaying his hands: "Here is one hand; here is another hand; therefore, there are at least two objects; consequently, external-world skepticism is refuted." This argument was formulated to uphold common sense and challenge skeptical positions. Subsequently, Ludwig Wittgenstein, in his posthumously published 1969 work On Certainty, contended that Moore's argument derived its force from the conventions of ordinary language use, rather than from any inherent epistemological insight.

Within contemporary philosophy, Richard Popkin emerged as a highly influential scholar concerning skepticism. His historical analysis of skepticism, presented in The History of Scepticism from Savonarola to Bayle (with its initial edition titled The History of Scepticism From Erasmus to Descartes), became the authoritative reference for modern scholarship in the field for decades following its 1960 publication. Barry Stroud also contributed extensively to the literature on philosophical skepticism, most notably with his 1984 monograph, The Significance of Philosophical Scepticism. Beginning in the mid-1990s, Stroud, in collaboration with Richard Fumerton, advanced significant anti-externalist arguments supporting a stance termed "metaepistemological scepticism." Other prominent contemporary philosophers recognized for their contributions to skepticism include James Pryor, Keith DeRose, and Peter Klein.

The historical development of skepticism within non-Western philosophical traditions.

Skepticism in Ancient Indian Philosophy

Ajñana

Ajñana (literally 'non-knowledge') represented a skeptical school within ancient Indian philosophy, functioning as a śramaṇa movement and a significant rival to early Buddhism and Jainism. Documented in Buddhist and Jain texts, Ajñana proponents asserted the impossibility of acquiring knowledge regarding metaphysical phenomena or ascertaining the truth value of philosophical propositions. They further argued that even if such knowledge were attainable, it would be both useless and disadvantageous for achieving ultimate salvation.

Buddhism

The historical Buddha affirmed certain doctrines, such as the possibility of nirvana; however, he also embraced a form of skepticism concerning specific questions, which he left 'un-expounded' (avyākata) or deemed 'incomprehensible' (acinteyya). Recognizing these predominantly metaphysical inquiries as unhelpful to the spiritual path and conducive to confusion and 'a thicket of views,' he advocated for the suspension of judgment regarding them. This strategy allowed him to forge an epistemic middle way, distinct from what he considered the extremes of claiming absolute objectivity (associated with the Jain Mahavira's assertions of omniscience) and extreme skepticism (exemplified by the Ajñana thinker Sanjaya Belatthiputta).

Later Buddhist philosophy maintained a high degree of skepticism regarding Indian metaphysical arguments. The Buddhist philosopher Nagarjuna, specifically, is considered the founder of the Madhyamaka school, which has been compared to Greek Skepticism. Nagarjuna's declaration of having 'no thesis' (pratijña) exhibits parallels with Sextus Empiricus's assertion of holding 'no position.' Nagarjuna famously initiates his magnum opus, the Mulamadhyamakakarika, by stating that the Buddha posited true happiness is attained by dispelling 'vain thinking' (prapañca, also known as 'conceptual proliferation').

Richard P. Hayes posits that the Buddhist philosopher Dignaga also exhibits characteristics of a skeptic, aligning with the general trajectory of early Buddhist philosophy. Hayes states:

...in both early Buddhism and in the Skeptics one can find the view put forward that man's pursuit of happiness, the highest good, is obstructed by his tenacity in holding ungrounded and unnecessary opinions about all manner of things. Much of Buddhist philosophy, I shall argue, can be seen as an attempt to break this habit of holding on to opinions.

Academics such as Adrian Kuzminski have proposed that Pyrrho of Elis (c. 365–270 BCE) may have encountered and been influenced by Indian Buddhists during his travels with Alexander the Great.

Cārvāka philosophy

The Cārvāka (Sanskrit: चार्वाक) school of materialism, also known as Lokāyata, constitutes a classically cited, yet historically disputed, school of ancient Indian philosophy. Despite the absence of surviving original texts or authoritative doctrines, proponents of this system are frequently referenced in the philosophical treatises of other schools, often serving as an initial counterpoint against which to articulate their own arguments.

Cārvāka is classified as a 'heterodox' (nāstika) system, characterized by its materialistic and atheistic philosophical orientation. This school was also recognized for its strong skepticism concerning the tenets of Indian religions, such as reincarnation and karma.

Jainism

While Jain philosophy posits the attainment of omniscience, or absolute knowledge (Kevala Jnana), at the moment of enlightenment, its theory of anekāntavāda, or 'many-sidedness'—also known as the principle of relative pluralism—permits a practical form of skeptical inquiry regarding philosophical and religious doctrines. This applies specifically to unenlightened beings, not to all-knowing arihants.

This theory posits that truth or reality is apprehended distinctly from various perspectives, implying that no singular viewpoint encompasses the entirety of truth. Jain doctrine asserts that an object possesses an infinite array of existential modes and qualities, which, owing to inherent human limitations, cannot be fully apprehended in all their aspects and manifestations. Anekāntavāda, literally meaning the doctrine of non-onesidedness or manifoldness, is frequently rendered as "non-absolutism." Syādvāda, the theory of conditioned predication, articulates anekānta by advocating the prefix "Syād" to every expression. Syādvāda functions not merely as an extension of Anekānta ontology but as an independent logical system. Given the intricate nature of reality, no solitary proposition can fully articulate its essence. Consequently, the term "syāt" should precede each proposition, imparting a conditional perspective and thereby mitigating any dogmatism within the statement. Jains believe that fully enlightened beings possess the capacity to perceive reality from all angles, thus attaining ultimate knowledge of all phenomena. This concept of omniscience, however, faced criticism from Buddhist thinkers like Dharmakirti.

Ancient Chinese Philosophy

Zhuang Zhou (circa 369 – circa 286 BCE)

Zhuang Zhou (莊子, "Master Zhuang"), a prominent ancient Chinese Taoist philosopher during the Hundred Schools of Thought period, articulated his skeptical perspectives through various anecdotes within the seminal work *Zhuangzi*, which is attributed to him.

• "The Debate on the Joy of Fish" (知魚之樂): This anecdote features Zhuang Zhou engaging in a discussion with his contemporary philosopher Hui Shi regarding their capacity to ascertain the happiness of fish in a pond. Zhuang Zhou famously remarked, "You are not I. How do you know that I do not know that the fish are happy?" (Autumn Floods 秋水篇, *Zhuangzi*).
• "The Butterfly of the Dream" (周公夢蝶): The "Butterfly Dream" paradox illustrates Zhuang Zhou's disorientation subsequent to dreaming he was a butterfly, leading to the reflection: "But he didn't know if he was Zhuang Zhou who had dreamt he was a butterfly, or a butterfly dreaming that he was Zhuang Zhou." (Discussion on Making All Things Equal 齊物篇, *Zhuangzi*).

Through these narratives in *Zhuangzi*, Zhuang Zhou conveyed his conviction regarding the inherent limitations of language and human communication, alongside the ultimate inaccessibility of universal truth, thereby establishing his identity as a skeptic. Nevertheless, he was not a radical skeptic; his application of skeptical methodologies was partial, primarily serving to substantiate his Taoist convictions, which he himself held dogmatically.

Wang Chong (27 – c. 100 CE)

Wang Chong (王充) emerged as the principal exponent of the skeptical faction within the Confucian school in China during the first century CE. He pioneered a methodology of rational critique, which he applied to challenge the pervasive dogmatic thought of his era, including phenomenology (the dominant contemporary Confucian ideology that correlated all natural phenomena with human ethics), state-sponsored cults, and popular superstitions. His philosophical framework integrated both Taoist and Confucian perspectives, grounded in a secular, rational approach to formulating hypotheses derived from natural occurrences to elucidate the cosmos. This approach exemplified a form of naturalism akin to the philosophical tenets of Epicureans such as Lucretius.

Medieval Islamic Philosophy

The Incoherence of the Philosophers, a treatise authored by the esteemed scholar Al-Ghazali (1058–1111), signifies a pivotal shift in Islamic epistemology. Ghazali's engagement with skepticism culminated in his adoption of a theological occasionalism, a doctrine asserting that all causal events and interactions arise not from material concatenations but from the direct and immediate volition of God.

Ghazali's late-life autobiography, The Deliverance From Error (Al-munqidh min al-ḍalāl ), details his intellectual journey. He describes resolving a crisis of epistemological skepticism through a divine illumination, which he termed "a light which God Most High cast into my breast...the key to most knowledge." Subsequently, he thoroughly investigated and comprehended the tenets of Kalam, Islamic philosophy, and Ismailism. While acknowledging the merits of Kalam and Islamic philosophy, Ghazali ultimately concluded that all three methodologies were insufficient, finding profound truth exclusively in the mystical experiences and spiritual insights derived from Sufi practices. William James, in Varieties of Religious Experience, recognized this autobiography as a significant text for "the purely literary student who would like to become acquainted with the inwardness of religions other than the Christian," drawing parallels to personal religious confessions and autobiographical works within the Christian tradition.

References

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