Rene Descartes is generally thought to be the originator of what we now call foundationalism in epistemology. However, while our favorite French rationalist should continue to hold this privileged status, it is precisely his foundationalist epistemology that is no longer tenable.
Pojman, in What Can we Know?, explains that Descartes believes there are exactly two ways of achieving a state of knowledge: intuition and deductive reasoning. While this methodology can also be heavily questioned, it is how Descartes arrives at this foundational truth that is in doubt here. Certain self-evident truths are seen by Descartes to be justified. He sees them as both intuitive and indubitable; and, therefore, we can be completely certain about these foundations. It is from here that we use deductive reasoning to achieve scientific knowledge in order to fill out our system of knowledge. All of these posterior beliefs are justified by the certainty of our foundations. And here we approach Descartes’ dilemma.
Descartes believes that all of our empirically-based, and deductively reasoned, beliefs are warranted because God would never develop our senses in such a way as to trick us (Cartesian demons aside). However, it is these very same empirical beliefs that Descartes uses to justify his belief that God exists in the first place. Therefore, our line of reasoning is, then, obviously circular and in need of some sort of modification, if not complete overhaul.