Hume's object, like that of Berkeley, is the improvement of Locke's doctrine of knowledge. In several respects he does not go so far as Berkeley, in others very much farther. In agreement with Berkeley's ultra-nominalism, which combats even the possibility of abstract ideas, he yet does not follow him to the extent of denying external reality. On the other hand, he carries out more consistently Berkeley's hint that immediate sensation includes less than is ascribed to it (_e.g._, that by vision we perceive colors only, and not distance, etc.), as well as his principle--destructive to the certainty of our knowledge of nature--that there is no causality among phenomena; and brings the question of substance to, the negative conclusion, that there is no need whatever for a support for groups of qualities, and, therefore, that substantiality is to be denied to immaterial as well as to material beings. The points in Locke's philosophy which seemed to Hume to need completion were different from those at which Berkeley had struck in. The antithesis of rational and empirical knowledge is more sharply conceived; the combination of ideas is not left to the choice of the understanding but placed under the dominion of psychological laws; and to the distinction between outer and inner experience (to the former of which priority is conceded, on the ground that we must have had an external sensation before we can, through reflection, be conscious of it as an internal phenomenon), there is added a second, as important as the other and crossing it, between impressions and ideas, of which the former are likewise made prior to the latter.