The great war between Cartesian Rationalism and Empiricism has been fought on several fronts. Empiricists believe that all concepts, meanings, and knowledge arise from experience, while Cartesian Rationalists defend the innateness of at least some concepts, meanings, and knowledge. Empiricists also usually believe that perception is a noncognitive state that serves as a foundation for cognitive states, while Cartesian Rationalists believe that perception is itself cognitive. Undoubtedly, there are other differences between Empiricism and Cartesian Rationalism, but these are the salient ones for the purposes of this book. In the seventeenth and eighteenth centuries, the battle was mainly over the issue of whether perception is constructive, proposition-like, and cognitive, on the one hand, or passive, image-like, and often noncognitive, on the other. Rationalists (as did Descartes in his Second Meditation discussion of the wax) defended the former while many Empiricists have argued for the latter position. Indeed, the British Empiricists, beginning with Locke and culminating in Hume, argued that perception is basically a phenomenal state, totally passive and unconstructed.

The British Empiricists were, however, Cartesian in one respect: they accepted Descartes' Internalism, his view that contents (including perceptual contents) are wholly in the mind. They simply believed that phenomena are the bearers of content, while Descartes thought that only proposition-like states are bearers of content. In the twentieth century, the debate shifted significantly, though these early differences remain large.