As the third form of causality was enumerated 'motive' for the will; but in that classification it was viewed from without, as belonging to the world of objects. Through the direct knowledge we have of our own will, we know also from within this determination by the presentation we call a motive. Hence emerges the fourth form of the principle of sufficient reason. This at a later stage makes possible the transition from physics to metaphysics. All these forms alike are forms of necessary determination. Necessity has no clear and true sense but certainty of the consequence when the ground is posited. All necessity therefore is conditional. In accordance with the four expressions of the principle of sufficient reason, it takes the fourfold shape of physical, logical, mathematical, and moral necessity. The sharp distinction between logical and mathematical truth, with the assignment of the former to conceptual and of the latter to intuitive relations, comes to Schopenhauer directly from Kant. So also does his view that the necessary form of causation is sequence; though here his points of contact with English thinkers, earlier and later, are very marked. Only in his statement of the 'law of motivation' as 'causality seen from within' does he hint at his own distinctive metaphysical doctrine. Meanwhile, it is evident that he is to be numbered with the group of modern thinkers who have arrived in one way or another at a complete scientific phenomenism. Expositors have noted that in his earlier statements of this he tends to lay more stress on the character of the visible and tangible world as mere appearance. The impermanence, the relativity, of all that exists in time and space, leads him to describe it, in a favourite term borrowed from Indian philosophy, as Maya, or illusion. Later, he dwells more on the relative reality of things as they appear. His position, however, does not essentially alter, but only finds varying expression as he turns more to the scientific or to the metaphysical side. From Hume's view on causation he differs not by opposing its pure phenomenism, but only by recognising, as Kant does, an _a priori_ element in the form of its law. German critics have seen in his own formulation an anticipation of Mill, and this is certainly striking as regards the general conception of the causal order, although there is no anticipation of Mill's inductive logic. On the same side there is a close agreement with Malebranche and the Occasionalists, pointed out by Schopenhauer himself. The causal explanations of science, he is at one with them in insisting, give no ultimate account of anything. All its causes are no more than 'occasional causes,'--merely instances, as Mill expressed it afterwards, of 'invariable and unconditional sequence.' From Mill of course he differs in holding its form to be necessary and _a priori_, not ultimately derived from a summation of experiences; and, with the