The Life of Reason by George Santayana — Read

Respectable tradition that human nature is fixed.

A conception of something called human nature arises not unnaturally on observing the passions of men, passions which under various disguises seem to reappear in all ages and countries. The tendency of Greek philosophy, with its insistence on general concepts, was to define this idea of human nature still further and to encourage the belief that a single and identical essence, present in all men, determined their powers and ideal destiny. Christianity, while it transposed the human ideal and dwelt on the superhuman affinities of man, did not abandon the notion of a specific humanity. On the contrary, such a notion was implied in the Fall and Redemption, in the Sacraments, and in the universal validity of Christian doctrine and precept. For if human nature were not one, there would be no propriety in requiring all men to preserve unanimity in faith or conformity in conduct. Human nature was likewise the entity which the English psychologists set themselves to describe; and Kant was so entirely dominated by the notion of a fixed and universal human nature that its constancy, in his opinion, was the source of all natural as well as moral laws. Had he doubted for a moment the stability of human nature, the foundations of his system would have fallen out; the forms of perception and thought would at once have lost their boasted necessity, since to-morrow might dawn upon new categories and a modified a priori intuition of space or time; and the avenue would also have been closed by which man was led, through his unalterable moral sentiments, to assumptions about metaphysical truths.