Descartes, but explored in more detail by Locke, Berkeley and Hume. If you don't know what I'm talking about, here's the 3-minute animated intro.
In today's lecture, Professor Millican provides a thought-provoking historical and conceptual analysis of this famous distinction, especially as it relates to the question of whether our perceptions can actually resemble objects out in the world. For Berkeley, the problem is that ideas and perceptions can resemble nothing but ideas and perceptions, and since these are not physical, then whatever perceptions are about cannot be physical either: good-bye material world. For Hume, what we have is more of a skeptical problem: if all we ever directly perceive are ideas in our minds (caused by perceptions), how can we know (and by what methods can we possibly demonstrate) that there's a world beyond those perceptions? As you can imagine, the answers to such questions will have a lot to say about the nature and limits of science itself...
Click here to see the course slides.
And check out Woody Allen's hilarious version of the homunculus problem.