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The position that reason has precedence over other ways of acquiring knowledge, or, more strongly, that it is the unique path to knowledge. It is most often encountered as a view in epistemology, where it is traditionally contrasted with empiricism, the view that the senses are primary with respect to knowledge. (It is important here to distinguish empiricism with respect to knowledge from empiricism with respect to ideas or concepts; whereas the former is opposed to rationalism, the latter is opposed to the doctrine of innate ideas.) The term is also encountered in the philosophy of religion, where it may designate those who oppose the view that revelation is central to religious knowledge; and in ethics, where it may designate those who oppose the view that ethical principles are grounded in or derive from emotion, empathy, or some other non-rational foundation.
The term ‘rationalism’ does not generally designate a single precise philosophical position; there are several ways in which reason can have precedence, and several accounts of knowledge to which it may be opposed. Furthermore, the very term ‘reason’ is not altogether clear. Often it designates a faculty of the soul, distinct from sensation, imagination, and memory, which is the ground of a priori knowledge. But there are other conceptions of reason, such as the narrower conception in which Pascal opposes reason to “knowledge of the heart” (Pensées, section 110), or the computational conception of reason Hobbes advances in Leviathan I.5.
The term might thus be applied to a number of philosophical positions from the ancients down to the present. Among the ancients, ‘rationalism’ and ‘empiricism’ especially denote two schools of medicine, the former relying primarily on a theoretical knowledge of the hidden workings of the human body, the latter relying on direct clinical experience. The term might also be used to characterize the views of Plato and later Neoplatonists, who argued that we have pure intellectual access to the Forms and general principles that govern reality, and rejected sensory knowledge of the imperfect realization of those Forms in the material world.
In recent philosophical writing, the term ‘rationalism’ is most closely associated with the positions of a group of seventeenth-century philosophers, Descartes, Spinoza, Leibniz, and sometimes Malebranche. These thinkers are often referred to collectively as the Continental rationalists, and are generally opposed to the socalled British empiricists, Locke, Berkeley, and Hume. All of the former share the view that we have a non-empirical and rational access to the truth about the way the world is, and all privilege reason over knowledge derived from the senses. These philosophers are also attracted to mathematics as a model for knowledge in general. But these common views are developed in quite differentways.
Descartes claims to take his inspiration from mathematics – not mathematics as commonly understood, but the analysis of the ancients. • According to Descartes, we start from first principles known directly by reason (the cogito ergo sum of the Meditations), what he calls intuition in his Rules for the Direction of the Mind; all other knowledge is deduced from there. A central aim of his Meditations is to show that this faculty of reason is trustworthy. The senses, on the other hand, are generally deceptive, leading us to mistake sensory qualities for real qualities of extended bodies, and leading us to the false philosophy of Aristotle and to Scholasticism.
Descartes does not reject the senses altogether; in Meditation VI he argues that the senses are most often correct in circumstances concerning the preservation of life. Perhaps paradoxically, experiment is important to Descartes’s scientific work. However, his primary interest is in the theoretical account of the phenomena experiment reveals, and while his position is unclear, he may have considered experiment as an auxiliary to intuition and deduction, or as a second-best method that can be used with problems too complex for pure reason. Malebranche, following Descartes, takes similar views in his Search after Truth, though unlike Descartes, he emphasizes original sin as the cause of our tendency to trust the senses.
Spinoza’s model for knowledge is Euclidean geometry, as realized in the geometrical form of the Ethics. Spinoza explicitly argues that we cannot have adequate ideas of the world through sensation (Ethics II, propositions 16–31). In the Ethics he does see a role for the senses in what he calls knowledge of the first and knowledge of the second kinds, and in the earlier Emendation of the Intellect, he suggests that the senses may be auxiliary aids to genuine knowledge. But the senses are imperfect and far less valuable, according to Spinoza, than intuition, i.e., knowledge of the third kind, from which sensory experience is excluded. • Spinoza’s rationalism is implicit in a central proposition of the Ethics, in accordance with which “the order and connection of ideas is the same as the order and connection of things” (Ethics II, proposition 7), allowing one to infer causal connections between bodies and states of the material world directly from the logical connections between ideas.
Leibniz, too, emphasizes reason over the senses in a number of ways. In his youth he believed that it would be possible to calculate the truth-value of every sentence by constructing a logical language whose structure mirrors the structure of relations between concepts in the world. This view is reflected in his mature thought in the doctrine that in every truth, the concept of the predicate is contained in the concept of the subject, so that if one could take the God’s-eye view (which, he concedes, we cannot), one could determine the truth or falsity of any proposition without appeal to experience (Discourse on Metaphysics, section 8). • Leibniz also argues that all truths are based on two basic principles, the law of non-contradiction (for necessary truths), and the principle of sufficient reason (for contingent truths) (Monadology, section 31), both of which can be known a priori.
And so, at least in principle, the truth-values of all propositions can be determined a priori. This reflects his practice in physics, where he derives a number of laws of motion from the principle of the equality of cause and effect, which can be known a priori on the basis of the principle of sufficient reason. But, at the same time, referring to the empirical school of ancient medicine, Leibniz concedes that “we are all mere Empirics in three fourths of our actions” (Monadology, section 28). • Each of the so-called Continental rationalists does, in his own way, privilege reason over the senses. But the common designation ‘Continental rationalism’ arose only much later, probably in the nineteenth century. For their contemporaries, more impressed with their differences than their common doctrines, the Continental rationalists did not form a single homogeneous school of thought.
A Priori • Prior to or independent of experience; contrasted with ‘a posteriori’ (empirical). These two terms are primarily used to mark a distinction between • (1) two modes of epistemic justification, together with derivative distinctions between • (2) kinds of propositions, • (3) kinds of knowledge, • and (4) kinds of argument. They are also used to indicate a distinction between • (5) two ways in which a concept or idea may be acquired.
