One way to state the conviction that all of these Enlightenment figures shared is that reality is not self-revealing. The way it shows up in ordinary experience is not to be taken seriously. For example, we see a blue dress, but “blue” isn’t in the dress; it is a mental state. Descartes and Locke both insisted on a distinction between “primary qualities,” which are properties of things themselves, and “secondary qualities,” which are a function of our own perceptual apparatus.
