According to this theory, there is both an _à priori_ and an _à posteriori_ element in our knowledge, or, to speak more accurately, there are both _à priori_ and _à posteriori_ conditions of our knowing, the _à posteriori_ condition being, as in all systems, individual experience, the _à priori_ condition being inherited mental aptitudes, which, as a rule, become more and more marked and persistent with each successive transmission. Now Locke lays stress simply upon the _à posteriori_ condition, though he recognizes a certain kind of _à priori_ condition in our "natural faculties," and the simple ideas furnished by reflecting on their operations. The very important condition, however, of inherited aptitudes facilitating the formation of certain general conceptions concurrently, or almost concurrently, with the presentation of individual experiences, did not occur to him as an element in the solution of the problem he had undertaken to answer, nor, in that stage of speculation, could it well have done so. His peculiar contribution to the task of solving this question consisted in his skilful and popular delineation of the _à posteriori_ element in knowledge, and in his masterly exposure of the insufficiency of the account of the _à priori_ element, as then commonly given. Locke's own theory was afterwards strained by Hume and Hartley, and still more by his professed followers in France, such as Condillac and Helvétius, till at last, in the opinion of most competent judges, it snapped asunder. Then, under the massive, though often partial and obscure, treatment of Kant, came the rehabilitation of the _à priori_ side of knowledge. In recent times, mainly by aid of the light thrown on it from other branches of inquiry, a more thorough and scientific treatment of psychology has done much, as I conceive, towards completing and reconciling the two divergent theories which at one time seemed hopelessly to divide the world of philosophic thinkers. And yet, as it appears to me, the ultimate mystery which surrounds the beginnings of intellectual life on the globe has by no means been removed.