The last of Locke's "primary qualities" which remain to be considered is mechanical solidity, or impenetrability. But our conception of this is derived from the sense of resistance to our own effort, or active force, which we meet with in association with sundry tactile or visual phenomena; and, undoubtedly, active force is inconceivable except as a state of consciousness. This may sound paradoxical; but let anyone try to realize what he means by the mutual attraction of two particles, and I think he will find, either, that he conceives them simply as moving towards one another at a certain rate, in which case he only pictures motion to himself, and leaves force aside; or, that he conceives each particle to be animated by something like his own volition, and to be pulling as he would pull. And I suppose that this difficulty of thinking of force except as something comparable to volition, lies at the bottom of Leibnitz's doctrine of monads, to say nothing of Schopenhauer's "Welt als Wille und Vorstellung;" while the opposite difficulty of conceiving force to be anything like volition, drives another school of thinkers into the denial of any connection, save that of succession, between cause and effect. * * * * * To sum up. If the materialist affirms that the universe and all its phenomena are resolvable into matter and motion, Berkeley replies, True; but what you call matter and motion are known to us only as forms of consciousness; their being is to be conceived or known; and the existence of a state of consciousness, apart from a thinking mind, is a contradiction in terms. I conceive that this reasoning is irrefragable. And therefore, if I were obliged to choose between absolute materialism and absolute idealism, I should feel compelled to accept the latter alternative. Indeed, upon this point Locke does, practically, go as far in the direction of idealism, as Berkeley, when he admits that "the simple ideas we receive from sensation and reflection are the boundaries of our thoughts, beyond which the mind, whatever efforts it would make, is not able to advance one jot."--Book II. chap, xxiii. § 29. But Locke adds, "Nor can it make any discoveries when it would pry into the nature and hidden causes of these ideas." Now, from this proposition, the thorough materialists dissent as much, on the one hand, as Berkeley does, upon the other hand. The thorough materialist asserts that there is a something which he calls the "substance" of matter; that this something is the cause of all phenomena, whether material or mental; that it is self-existent and eternal, and so forth.