The first modern rationalist was Descartes, an original mathematician whose ambition was to introduce into philosophy the rigour and clearness that delighted him in mathematics. In his Meditations on First Philosophy (1641), he set out to doubt everything in the hope of arriving in the end at something indubitable. This he reached in his famous dictum cogito ergo sum, “I think, therefore I am” (expressed in the Meditations as cogito sum, “I think, I am”); for to doubt one’s own doubting would be absurd. Here then was a fact of absolute certainty, rendered such by the clearness and distinctness with which it presented itself to his reason. His task was to build on this as a foundation, to deduce from it a series of other propositions, each following with the same self-evidence. He hoped thus to produce a philosophical system on which people could agree as completely as they do on the geometry of Euclid. The main cause of error, he held, lay in the impulsive desire to believe before the mind is clear. The clearness and distinctness upon which he insisted was not that of perception but of conception, the clearness with which the intellect grasps an abstract idea, such as the number three or its being greater than two.

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His method was adopted in essentials by both Benedict Spinoza (1632–77) and G.W. Leibniz (1646–1716), who agreed that the framework of things could be known by a priori thinking. They differed from him, however, in their starting points. What was most undeniable to Spinoza was not the existence of his self but that of the universe, called by him “substance.” From the idea of substance, and with the aid of a few definitions and axioms, he derived his entire system, which he set forth in his Ethics in a formal fashion patterned after Euclid’s geometry. Still, for both Spinoza and Leibniz much in nature remained stubbornly opaque. Leibniz distinguished necessary truths, those of which the opposite is impossible (as in mathematics), from contingent truths, the opposite of which is possible, such as “snow is white.” But was this an ultimate distinction? At times Leibniz said boldly that if only humans knew enough, they would see that every true proposition was necessarily true—that there are no contingent truths, that snow must be white.

How, then, does reason operate and how is it possible to have knowledge that goes beyond experience? A new answer was given by Kant in his Critique of Pure Reason (1781; 1787), which, as he said, involved a Copernican revolution in philosophy. The reason that logic and mathematics will remain valid for all experience is simply that their framework lies within the human mind; they are forms of arrangement imposed from within upon the raw materials of sensation. Humans will always find things arranged in certain patterns because it is they who have unwittingly so arranged them. Kant held, however, that these certainties were bought at a heavy price. Just because a priori insights are a reflection of the mind, they cannot be trusted as a reflection of the world outside the mind. Whether the rational order in which sensation is arranged—the order, for example, of time, space, and causality—represents an order holding among things-in-themselves (German Dinge-an-sich) cannot be known. Kant’s rationalism was thus the counterpart of a profound skepticism.

G.W.F. Hegel (1770–1831), the most thoroughgoing of rationalist thinkers, attempted to break out of this skepticism. He argued that to think of an unknowable is already to bring it within the sphere of what is known and that it is meaningless to talk of a region in which logic is invalid. Further, to raise the question “Why?” is to presume that there is an intelligible answer to it; indeed, the faith of the philosopher must be that the real is the rational and the rational real, for this faith is implicit in the philosophical enterprise itself. As an attempt to understand and explain the world, philosophy is a process of placing something in a context that reveals it as necessary. But this necessity is not, as earlier rationalists had supposed, an all-or-nothing affair issuing in a self-evident finality. Understanding is a matter of degree. What alone would wholly satisfy thought is a system that is at once all-inclusive and so ordered that its parts entail each other. Hegel believed that the universe constitutes such a whole and, as an idealist, held that it is a single, absolute mind. To the degree that philosophers embody and realize this mind, their own minds will achieve both truth and reality. Indeed, the advance of civilization reflects the enlarging presence and control of such a system in the human spirit. Broadly similar rationalistic systems were developed in England by F.H. Bradley (1846–1924) and Bernard Bosanquet (1848–1923) and in America by Josiah Royce (1855–1916).

Ethical rationalism

The views of Kant were presented above as typical of this position (see above Types and expressions of rationalism). But few moralists have held to ethical rationalism in this simple and sweeping form. Many have held, however, that the main rules of conduct are truths as self-evident as those of logic or mathematics. Lists of such rules were drawn up by Ralph Cudworth (1617–88) and Henry More (1614–87) among the Cambridge Platonists, who were noted for holding that moral principles were intrinsic to reality; later Samuel Clarke (1675–1729) and Richard Price (1723–91), defenders of “natural law” ethics, and the “common sense” moralist Thomas Reid (1710–96) also presented such lists. A 20th-century revision of this rationalism was offered by the intuitionists H.A. Prichard (1871–1947) and Sir David Ross (1877–1971) of Oxford under the name of deontology (from the Greek deon, “duty”), which respects duty more than consequences. Ross provides a list of propositions regarding fidelity to promises, reparation for injuries, and other duties, of which he says: “In our confidence that these propositions are true there is involved the same trust in our reason that is involved in our trust in mathematics.” What is taken as self-evident, however, is not specific rules of conduct but prima facie duties—the claims that some types of action have on humans because of their nature. If a person is considering whether to repay a debt or to give the money to charity, each act has a self-evident claim on that person, and their comparative strengths must be settled by a rational intuition.

The most-influential variety of 20th-century ethical rationalism was probably the ideal utilitarianism of the British moralists Hastings Rashdall (1858–1924) and G.E. Moore (1873–1958). Both were teleologists (from the Greek telos, “end”) inasmuch as they held that what makes an act objectively right is its results (or end) in intrinsic goods or evils. To determine what is right, reason is required in two senses: first, the inference to the consequences is an act of inductive reasoning; second, the judgment that one consequence is intrinsically better than another is a priori and self-evident. Moore thought that there is a single rule for all conduct—one should so act as to produce the greatest good—and that this is also a principle self-evident to reason.