The philosophers of the seventeenth and eighteenth centuries are often separated into rationalists and empiricists. While this distinction certainly blurs similarities between philosophers of both “schools”, this retrospective classification has some value at least in bringing out tendencies of the philosophers grouped under these headings. The contrast chiefly lies in what is said to be knowable by pure reason alone. Some factors consistently underlie rationalist philosophy.

Rationalism holds that the human mind has the capacity, logically speaking, to establish truths about the nature of reality (including ourselves) by reason alone independently of experience; indeed, if knowledge of the fundamental structure of the world in the proper scientific sense is possible, then it must be derived from reason, which alone has access to the required certain, necessary, universally valid, timeless truths; the senses inform us only of what is uncertain, contingent, particular, perspectival and transient. These necessary truths about the world can be known to be true merely through our properly understanding the concepts they involve or are deduced from such truths, and ideally they form a single deductive system. Truths known a priori by pure understanding, if they do not concern the world as it appears in perception, instead concern a really existing intelligible world that underlies the appearance of changing particulars that we experience; this underlying reality makes intelligible, and ultimately explains, the appearances. The intellect has access to concepts, and the terms that express them, whose meaning does not depend on being referred to some feature of our experience.