theoretical reason in its traditional sense, a faculty or capacity whose province is theoretical knowledge or inquiry; more broadly, the faculty concerned with ascertaining truth of any kind (also sometimes called speculative reason). In Book 6 of his Metaphysics, Aristotle identifies mathematics, physics, and theology as the subject matter of theoretical reason. Theoretical reason is traditionally distinguished from practical reason, a faculty exercised in determining guides to good conduct and in deliberating about proper courses of action. Aristotle contrasts it, as well, with productive reason, which is concerned with ‘making’: shipbuilding, sculpting, healing, and the like.
Kant distinguishes theoretical reason not only from practical reason but also (sometimes) from the faculty of understanding, in which the categories originate. Theoretical reason, possessed of its own a priori concepts (‘ideas of reason’), regulates the activities of the understanding. It presupposes a systematic unity in nature, sets the goal for scientific inquiry, and determines the ‘criterion of empirical truth’ (Critique of Pure Reason). Theoretical reason, on Kant’s conception, seeks an explanatory ‘completeness’ and an ‘unconditionedness’ of being that transcend what is possible in experience.
Reason, as a faculty or capacity, may be regarded as a hybrid composed of theoretical and practical reason (broadly construed) or as a unity having both theoretical and practical functions. Some commentators take Aristotle to embrace the former conception and Kant the latter. Reason is contrasted sometimes with experience, sometimes with emotion and desire, sometimes with faith. Its presence in human beings has often been regarded as constituting the primary difference between human and non-human animals; and reason is sometimes represented as a divine element in human nature. Socrates, in Plato’s Philebus, portrays reason as ‘the king of heaven and earth.’ Hobbes, in his Leviathan, paints a more sobering picture, contending that reason, ‘when we reckon it among the faculties of the mind, . . . is nothing but reckoning – that is, adding and subtracting – of the consequences of general names agreed upon for the marking and signifying of our thoughts.’ See also PRACTICAL REASON, RATIONALIT. A.R.M.