The expression “clarity and distinctness,” including its adjectival and adverbial forms, is used in the literature, especially most recently, with the frequency and reverence befitting a mantra. Cogito ergo sum appears on t-shirts and in New Yorker cartoons, and well it might, since it is by Descartes’ own account the fulcrum of his whole system. But, to change the metaphor somewhat, the system is generally taken to be driven by clarity and distinctness – the goal of the Cartesian method, which begins by applying doubt to the obscure and confused and ends with the certainty of the clear and distinct. In short, clarity and distinctness is what Descartes’ philosophy is supposed to be all about.

The importance assigned to clarity and distinctness appears as follows. Seeking unshakable certainty about what cannot conceivably be false, Descartes arrives at the knowledge of his own existence, which cannot be challenged even by the supposition of an evil demon (see doubt). Secure in this knowledge, he asks what it is that makes this knowledge so certain and replies, near the outset of the Third Meditation, that in it “there is simply a clear and distinct perception of what I am asserting.” Since what he is then perceiving could never turn out to be false, at least not while he is perceiving it, he has what is required for certainty of its truth. He then draws a momentous conclusion, which is the engine of his method thereafter. “So I now seem to be able to lay it down as a general rule that whatever I perceive very clearly and distinctly is true” (AT VII 35, CSM II 24). The task then is simply to find what is clear and distinct, which is Descartes’ double-barreled criterion of truth.

Given the importance assigned to “clarity and distinctness,” it is surprising that relatively little attention has been given to what Descartes means by clarity and distinctness. Also surprising, only once in his entire corpus does Descartes make an explicit effort to tell us. In the Principle of Philosophy, Descartes reiterates the truth rule from the Third Meditation (in the somewhat weaker form that we can avoid error by assenting only to what we clearly and distinctly perceive) and indicates how we often go wrong with respect to that rule.