A philosophical attitude that questions the reliability or even the possibility of acquiring knowledge about the world. Skepticism deeply influenced early-modern discussions about scientific method. Skeptics argue that neither the senses nor a priori reasoning are reliable sources of knowledge about the world. Skepticism, in the Western tradition, has its roots in Greek philosophy. There were two schools of Greek skepticism. Academic skepticism, which developed within Plato’s (428–348 B.C.E.) Academy, maintained that nothing can be known. Pyrrhonian skepticism, formulated by Pyrrho of Elis (ca. 360–275 B.C.E.) and further developed in Alexandria during the first century B.C.E., received its fullest development in the writings of Sextus Empiricus (ca. 200). He criticized the academic skeptics for holding the negative dogmatic view that we can know nothing. He questioned whether we can even know whether we know anything and recommended the suspension of belief. In The Outlines of Pyrrhonism and Against the Mathematicians, Sextus laid out the Pyrrhonian arguments, known as tropes, in a systematic attack on dogmatic claims to knowledge. Each of these modes, or tropes, was designed to show that evidence from the senses is an unreliable source of knowledge about the real natures of things because it leads to contradictory conclusions about the observed object. Serious consideration of skepticism revived during the Renaissance with the recovery of the writings of the ancient skeptical writers. Coupled with the intellectual crisis of the Reformation, this revival led to a general skeptical crisis in European thought, which prompted many natural philosophers to question the foundations of knowledge about the world and traditional methods for seeking it.
Skepticism played two roles in seventeenth-century natural philosophy: it provided a powerful tool for criticizing traditional Aristotelian methodological prescriptions that had outlined ways of discovering certain truths about the real essences of things, and it led to the formulation of new approaches to knowledge and method. René Descartes (1596– 1650) applied the skeptical arguments to all forms of knowledge, arguing that traditional methods did not provide any kind of epistemological warrant for claiming certainty. He used this method of systematic doubt to root out all dubitable claims in his search for a solid foundation upon which to build natural philosophy. In his Discourse on Method (1637) and Meditations (1641), he showed how claims based on empirical methods and even the results of mathematical demonstration could be doubted in light of skeptical critique.
Descartes believed that his new method could overcome skeptical doubts. He was determined to find some indubitable proposition upon which he could build a natural philosophy that would provide certain knowledge about the real essences of things. Descartes thought that he had found such a proposition in his famous “Cogito ergo sum” (I think, therefore I am). Starting from the indubitable cogito, he attempted to prove the existence of God, whose necessary veracity provided an epistemological warrant for reasoning from ideas in his mind to the nature of things in the world. On this basis, he claimed that anything we perceive clearly and distinctly exists in the world precisely in the way that we perceive it. Descartes believed that he could proceed to certain conclusions by means of geometrical demonstration. He described his natural philosophy in detail in the Meditations and The Principles of Philosophy (1644). Pierre Gassendi (1592–1655) and Marin Mersenne (1588–1648) deployed skepticism to formulate a very different approach to natural philosophy. In his first published work, Exercitationes paradoxicae adversus Aristoteleos (Paradoxical Exercises Against the Aristotelians, 1624), Gassendi closely followed the tropes of Sextus Empiricus to disprove the possibility science in the Aristotelian sense. Without the ability to reason from observations to the essential attributes of things, Gassendi considered Aristotle’s (384–322 B.C.E.) method to be worthless. Gassendi thus redefined the epistemic goal of science, replacing certainty with probability. He argued that knowledge consists of probable statements based on our experience of the phenomena. He denied the possibility of acquiring knowledge of the essences of things, thus rejecting the traditional Aristotelian and Scholastic conception of scientia as demonstrative knowledge of real essences and replacing it with what he called a “science of appearances,” probable knowledge of the appearances of things. During the second half of the seventeenth century, a group of English natural philosophers associated with the early Royal Society elaborated this epistemology of empirical knowledge cast in terms mitigated skepticism—as Richard Popkin (1979) has called this view—into an account of the degrees of certainty it can achieve. They denied that every kind of knowledge can attain the certainty of mathematical demonstration.
Only God possesses knowledge that is absolutely and infallibly certain. Mathematical certainty pertains to mathematics and the parts of metaphysics that can be established by logic and mathematical demonstration that compel assent. Moral certainty, which characterizes knowledge that is based on immediate sense experience or introspection, lies a step below mathematical certainty. A slightly weaker kind of moral certainty characterizes belief and conclusions about ordinary life that are based on observation and the testimony of others. Finally, opinions based on second-hand reports of sense observations can be known only as probable or perhaps as just plausible. Robert Boyle (1627–1691) elaborated the theory of degrees of certainty into an empiricist epistemology for natural philosophy. He said that theories should be evaluated in terms of their intelligibility, simplicity, explanatory scope, and predictive power and that they are confirmed to the degree that they successfully explain different kinds of observed facts. He noted that intelligibility to a human understanding is not necessary to the truth or existence of a thing. Thus, he believed that the results of natural philosophy could at best attain physical certainty (i.e., a high degree of probability). John Locke (1632–1704) drew on the tradition of mitigated skepticism and degrees of certainty to articulate a fully developed empiricist epistemology. He claimed that all of our ideas originate from either the senses or reflection on ideas drawn from the senses. He denied that we can attain certainty about things in the world and that we can acquire knowledge of the real essences of things. Acknowledging that this approach represented a major departure from the epistemic goals of both the Aristotelian and Cartesian approaches to natural philosophy, he asserted “that natural philosophy is not capable of being made a science.”Tags: Academic skepticism, Greek philosophy, Greek skepticism