To say that an epistemology is a form of rationalism is to say that it is a theory of knowledge which argues that the ultimate justification for any claim to know something is to show that what is known is dictated by or supported by reason.  For example if it is true by definition that all whales are mammals and that all mammals have lungs, I can know by reasoning alone, that the belief that all whales have lungs must be true.  This claim follows by valid deductive reasoning from its premisses. Rationalist theories of knowledge thus make reason the foundation on which the justification for all cognitive claims ultimately rests.  Since philosophers use the word "mind" to refer to that which has the faculty of reasoning, another way to put this is to say that rationalist epistemologies make the mind the source of knowledge.

From an epistemological point of view the set of problems which come to characterize the period of "modern" philosophy that runs from roughly 1600 to 1900 are largely derived from the orientation given to philosophy by the philosophy of Rene Descartes. Customary pedagogy treats Descartes as the founder of "continental rationalism" -a tradition which runs through Spinoza and Leibniz all the way to Kant, and afterwards to the 19th century German idealists. In contrast to the rationalist tradition is generally counterposed the "British empiricist" tradition beginning with Locke, running through Berkeley and Hume to Mill in the 19th century. Empiricists disagree with rationalists that reason provides a foundation for justification and argue that ultimately all justification of cognitive claims rests on what the sensory experience tells us. However, the tendency to pit these two philosophical traditions against each other obscures the fact that they also shared many epistemological presuppositions in common, and for the most part these were already present in the work of Descartes.

In contrast the Aristotle's dictum that "philosophy begins in wonder" and that the search for "theoretical knowledge" -knowledge for its own sake- is a response to such wonder, for Descartes philosophy begins in doubt, a response to the common Enlightenment belief that much of what had been handed down from the Ancients was not knowledge and needed to be swept away before a proper foundation for genuine knowledge could begin.

In spite of wanting to get beyond Aristotle, Descartes' ideal for the complete system of all knowledge (all certainty), what he would call "science," is a deductive system based on the model of mathematics or geometry, as inherited from the ancients. Such a system would begin with axioms that are necessarily true because they have been established as beyond any logically possible doubt by Descartes' methodological doubting. All other true judgments would then be deduced, like the "theorems" of geometry from these certain axioms. As long as all deductions are valid, if the starting axioms (premisses) are certain, all else that follows from them is equally certain. But Descartes' starting axioms are not Aristotle's "basic truths" expressing the essential natures of primary substances as required by Aristotelian science, nor are they arrived at by abstracting the universal from sensory experience of the particular primary substances.

Descartes' idea of starting afresh was underwritten by what was perceived as the "new science" of the day and came to be known as "the mechanistic world-view." By the standards of what came to be counted as an acceptable scientific explanation, the science of Aristotle appeared as nothing but playing with words, and conversely by the standards of Aristotelian science which insisted in final causes or purposes, the mechanical "explanations" of the new science were hardly an adequate response to Aristotelian wonder. Nevertheless, the modern project came into being originally with the attempt to formulate a theory of knowledge that would justify the claims of this new mechanistic mode of explanation to be genuine scientific knowledge. From Descartes through Kant all the modern philosophers, whether rationalists or empiricists, shared this common goal, though at least in the case of Hume it was regarded as an unattainable one.