 A foundationally justified belief (henceforth simply: ‘foundational belief’) is one that does not depend on any other beliefs for its justification. According to foundationalism, any justified belief must either be foundational or depend for its justification, ultimately, on foundational beliefs. Historically, foundationalism was very widely, almost universally accepted. Aristotle argued that “not all knowledge is demonstrative” (i.e., not all knowledge is based on an argument from other things known), and that some knowledge must be “independent of demonstration” (Posterior Analytics, I.3). Many of the medieval philosophers seemed in agreement with Aristotle, holding that all knowledge must rest on “first principles” or “self-evident truths” of some sort. More recently, Descartes famously held that all knowledge must rest on a secure foundation of indubitable truths (see the entry on  Descartes’s epistemology).  Many other philosophers of the early modern period, including Locke, Leibniz, Berkeley, Hume, and Reid, all seemed to accept foundationalism as well, despite disagreeing about much else. When an argument was implicitly or explicitly offered for the view, it was most often some version of the now famous epistemic regress argument. (See Aristotle’s Posterior Analytics, I.3, for an early version of this argument.) Before presenting the argument we should discuss a principle it depends on.