 Descartes’ contrast between ideas that are clear and distinct as opposed to ideas that are obscure and confused is central to the Logic. It is primarily concerned with the ways in which thought arrives at the truth; the epistemic status of the ideas with which one thinks is more basic than rules for proceeding from one thought to another (Gaukroger 1989, Nelson 2017). Although Descartes is usually read as considering the clarity and distinctness of an idea to be a psychological criterion external to the idea itself (but see Lennon 2008: 137–48), the Logic agrees with Spinoza that perceptions of unconfused, true ideas are known as such without recourse to a criterion of the sort sought by ancient sceptics. A good example of the employment of clarity and distinctness is provided by the characterization of the attributes and modes of substances. Again following Descartes, according to the Logic an attribute is recognized by one’s inability to distinctly perceive a substance if one of its attributes is excluded from it. Extension, for example, is an attribute of bodies. Bodies cannot be distinctly perceived except as extended. A mode cannot be distinctly perceived except in relation to the substance of which it is a mode, but substances can be distinctly perceived while some particular modes are excluded from them. A mind can be distinctly perceived without the thought of a particular pain, but that particular thought is only confusedly perceived unless it is perceived as that mind’s thought.