Following the publication of René Descartes’ Meditations on First Philosophy in 1641, Marin Mersenne solicited objections from noted philosophers, including Pierre Gassendi. Gassendi presented a number of radically skeptical objections to the rationalism of Descartes, including several addressing Descartes’ Third Meditation, which deals with the existence of God. The first of these objections deals with the concept of “clear and distinct” ideas, upon which Descartes relies as necessarily true.
Such ideas are the subject of Descartes’ own second paragraph, which involves a somewhat obfuscated logical argument. In it, Descartes begins with the one certainty he derived from his Second Meditation, “that I am a thinking thing.” The only source of this certainty, he now writes in his Third Meditation, is his clear and distinct perception of his own thought, so it would be undermined if any of his clear and distinct ideas were false. Because he is in fact certain, he concludes that “whatever I perceive very clearly and distinctly is true.”
Descartes thus makes a radical inductive jump from his perception of cognition to the truth of all clear and distinct ideas. This jump is logically complicated and seems to involve a few tricks. Descartes claims that he is certain of his cognition only because of his clear and distinct perception of it, for instance, but he in fact has the arguments of his Second Meditation—in which he demonstrates exactly that—to build upon. If Descartes’ knowledge stems from the arguments of his previous meditation, it is unclear whether clarity and distinctness in fact guarantee truth, even within the broader shape of Descartes’ argument.
The manipulative bounds of this argument hinge on Descartes’ second premise, the claim that a “clear and distinct perception… wouldn’t be enough to make me certain of its truth if it could ever turn out that something that I perceived so clearly and distinctly was false.” The hidden assumption here is that Descartes could somehow tell if one of his clear and distinct perceptions were false; if any of them “could ever turn out to be” false, he somehow knows they are now and can immediately cease to trust in clarity and distinctness. Without this unfounded assumption, Descartes demonstrates only how to disqualify clear and distinct perceptions from automatic truth; he does not make a functional positive argument for the truth of such perceptions.
Gassendi presents an opposing argument that is both compelling and clever: “the only thing that we can consider as clearly and distinctly perceived and therefore infer to be true,” he writes, “is that if something appears to anyone to the the case then it appears to be the case.” He essentially denies the epistemic value of clear and distinct perceptions, accepting them as sources of knowledge only when they take the form of tautologies. As an example, Gassendi writes that the clear and distinct flavor he perceived when eating a melon changed over time, implying that not all such perceptions can be true representations of the real flavor of the melon, even though they are true representations of themselves.
Similarly, Gassendi began to question the axioms of mathematics when he read arguments against a few of the central principles of Euclidean geometry. Instead of merely shifting his trust to the new axioms he discovered, however, he reached a state of doubt with regard to “the nature of mathematical propositions,” which he extended to philosophy in his objections to Descartes. This justification for doubt is particularly important because prominent 17th century philosophers such as Descartes, Thomas Hobbes, and Benedict Spinoza modeled their philosophical writings on the structure of Euclidean geometric texts, sometimes even presenting propositions argued from axioms.
This distrust of the sensation of clarity, which Gassendi traces to “the arguments of the sceptics,” seems opposed to the specific structure of Descartes’ meditation but not, perhaps, to its spirit, which begins by promoting exactly this sort of radical doubt. Descartes himself states at the beginning of the meditation that he would “regard all mental images of bodily things as empty, false and worthless,” so all Gassendi needed to argue was that this skepticism should be extended from perceptions of things to perceptions of ideas. Descartes’ one-sentence response to this argument, published as part of his responses to objections in some editions of his works, unfortunately leaves much to be desired: “Your next point, taken from the skeptics,” he writes, “is a standard move, and not a bad one, but it proves nothing.”
Despite Descartes’ low opinion of it, Gassendi’s objection seems to perceive several of the flaws in the first paragraphs of the Third Meditation. Because this meditation goes on to demonstrate the existence of God from Descartes’ professed poverty of assumptions, a search for hidden assumptions within the text is critical to a thorough understanding of Descartes’ arguments and an interrogation into their validity. Central among these assumptions are those supporting the claim that clear and distinct perceptions are always true.