The foundations of idealism in the modern sense were laid by the thinkers who sought breathing room for mind and will in a deeper analysis of the relations of the subject to the world that it knows. From the outset English philosophy had a leaning to the psychological point of view, and Locke was only carrying on the tradition of his predecessors and particularly of Hobbes in definitely accepting it as the basis of his _Essay_. It was, however, Berkeley who first sought to utilize the conclusions that were implicit in Locke's starting-point to disprove "the systems of impious and profane persons which exclude all freedom, intelligence, and design from the formation of things, and instead thereof make a self-existent, stupid, unthinking substance the root and origin of all beings." Berkeley's statement of the view that all knowledge is relative to the subject--that no object can be known except under the form which our powers of sense-perception, our memory and imagination, our notions and inference, give it--is still the most striking and convincing that we possess. To have established this position was a great step in speculation. Henceforth ordinary dogmatic dualism was excluded from philosophy; any attempt to revive it, whether with Dr Johnson by an appeal to common prejudice, or, in the more reflective Johnsonianism of the 18th-century Scottish philosophers, must be an anachronism. Equally impossible was it thenceforth to assert the mediate or immediate certainty of material substance as the cause either of events in nature or of sensations in ourselves. But with these advances came the danger of falling into error from which common-sense dualism and naturalistic monism were free. From the point of view which Berkeley had inherited from Locke it seemed to follow that not only material substance, but the whole conception of a world of objects, is at most an inference from subjective modifications which are the only immediately certain objects of knowledge. The implications of such a view were first clearly apparent when Hume showed that on the basis of it there seemed to be nothing that we could confidently affirm except the order of our own impressions and ideas. This being so, not only were physics and mathematics impossible as sciences of necessary objective truth, but our apparent consciousness of a permanent self and object alike must be delusive. Kant. It was these paradoxes that Kant sought to rebut by a more thoroughgoing criticism of the basis of knowledge the substance of which is summed up in his celebrated Refutation of Idealism,[1] wherein he sought to undermine Hume's scepticism by carrying it one step further and demonstrating that not only is all knowledge of self or object excluded, but the consciousness of any series of impressions and ideas is itself impossible except in relation to some external permanent and universally accepted world of objects. Leibnitz.