Hume vs. Descartes: Does Hume ever adequately respond to Descartes?

Hume gets idolized today on the Internet as sort of the paradigmatic rational human: empiricist, naturalist, atheist, a thinker who relies solely on skepticism and observation and eschews metaphysics and religion. You can find countless references to Hume's fork on blogs and forums today along with sentiments like "Hume destroyed metaphysics."

But from reading Hume I don't really see how or where Hume actually does manage to do this. Hume is an incredible philosopher and his vision and analysis and writing are pretty awesome but I can't help feel that he never really responded to rationalists like Descartes or set his own empiricist philosophy on any kind of secure foundation the way Descartes did. So let's consider the two philosophers on a critical topic: knowledge and existence.

By the end of Meditation II Descartes has basically established what he wanted to. I've never come across any philosopher who doesn't believe Descartes has definitively established a universal relation between thinking and existing and a counter to global skepticism, even if they disagree about the details of the subject of existence and of course what Descartes further inferred from this relation. But by Hume's account Descartes never should have been able to do this. Descartes explicitly tosses out both sense experience and any notion of corporeal things that comes from his imagination, yet he is able to establish a matter of fact about the world that he did not know before.

By contrast, both the positive and negative aspects of Hume's own philosophy rely on his copy principle and the relation of simple ideas to simple impressions of sense perception. Hume never gives an adequate account of what a simple impression is: a color can have hue or luminance or intensity so is it a simple impression or complex? If we reject the copy principle then the whole basis for assertions like Hume's fork falls apart. Even Hume was aware of how inadequate this principle was. If Hume's account is correct when I look at a color wheel in all the places where shades of color exist that I've never seen before there should be a blank. Since my mind has never tagged that particular shade with the name 'blue' or 'green' there is no way I could know this 'simple' sense impression is this color purely by using my imagination or reason.

Hume's empiricist theory of ideas is inadequate but this is the entire foundation on which his rejection of metaphysics and purely rational knowledge like what Descartes is doing rests. Hume doesn't even believe we can actually reason for the continued existence of something, and what beliefs we have regarding things like causality and continued existence are not from reason. Yet Descartes seems to have no problem establishing a sufficient condition for the continued existence of something, without recourse to sense experience or imagination, use only his reason.

The entire negative part of Hume's philosophy is based on his copy principle. The copy principle itself can be shown to be inadequate, but Descartes doesn't even need the copy principle to be false in his argument which precedes Hume by a century. It is almost as if Descartes is writing in response to Hume and not the other way around. Hume says our knowledge of matters of fact must be grounded purely in sense experience. Descartes says "alrighty lemme discard everything that I can possibly know through sense experience and see what I can still find."


So what really is Hume's response to Descartes' argument that yes there is knowledge we can acquire using pure rational reflection?  Hume's rebuttal can't just be Descartes isn't actually acquiring knowledge about matters of fact -- this is just question-begging.

Although Hume's influence on epistemology and reputation today are well deserved, I can't help feel the popular conception of Hume as a destroyer of metaphysics is wildly unfounded.