Nor do we fare better if we attempt to find unity in the world of nature as a whole. Nature has its existence as a whole in space and time. Indeed, it is only a way of expressing the totality of phenomena of space and time. It is a mere aggregate, a collection. Its very essence is plurality, difference. It is divisible without limit, and each of its divisions has as good a right to be called one as the whole from which it is broken off. We shall consider hereafter Leibniz's idea of infinity; but it is easy to see that he must deny any true infinity to nature. An ultimate whole made up of parts is a contradictory conception; and the idea of a quantitative infinite is equally so. Quantity means number, measure, limitation. We may not be able to assign number to the totality of occurrences in nature, nor to measure her every event. This shows that nature is indefinitely greater than any _assignable_ quantity; but it does not remove her from the category of quantity. As long as the world is conceived as that existing in space and time, it is conceived as that which has to be measured. As we saw in the last chapter, the heart of the mechanical theory of the world is in the application of mathematics to it. Since quantity and mathematics are correlative terms, the natural world cannot be conceived as infinite or as an ultimate unity. In short, Leibniz urges and suggests in one form and another those objections to the mechanical theory of reality which later German philosophers have made us so familiar with. The objections are indeed varied in statement, but they all come to the impossibility of finding any unity, any wholeness, anything except plurality and partiality in that which is externally conditioned,--as everything is in nature. But the reasons as thus stated are rather negative than positive. They show why the ultimate unity cannot be conceived as material, rather than why it must be conceived as spiritual. The immediate evidence of its spiritual nature Leibniz finds in the perception of the one unity directly known to us,--the "me," the conscious principle within, which reveals itself as an active force, and as truly one, since not a spatial or temporal existence. And this evidence he finds confirmed by the fact that whatever unity material phenomena appear to have comes to them through their perception by the soul. Whatever the mind grasps in one act, is manifested as one.