Duality allows Descartes to identify relational structures between opposites so that the whole can be better comprehended. To explain his understanding of dichotomy, Descartes states, “For from the fact that I cannot conceive a mountain without a valley, it does not follow that there is any mountain or any valley in existence, but only that the mountain and the valley, whether they exist or do not exist, cannot in any way be separated from the other” (24). Descartes argues that to know a mountain one must also know a valley or, rather, that the two are intertwined because they are opposites. Further, the two are so connected that it is only through meditation that they appear separate.

Descartes applies this understating to his investigation of substance or the composite of matter and form. Substances, he argues, are dualistic by nature, adhering to three major dichotomies: external/internal, particular/universal, and finite/infinite. This duality moves Descartes to assert that composites are more knowable than finite substances as they retain a level of objectivity. In the wax experiment, Descartes posits appearances as susceptible to change and thus deceptive. Objectivity, by contrast, is what allows for continuity amid change.