In particular, we must carefully avoid falling into the mistake of thinking of the Reality and the world of its appearances as though they formed two distinct realms. In a systematic unity, we must remember, the whole can exist only in so far as it expresses its nature in the system of its parts, and again the parts can have no being except as the whole expresses itself through them. To the degree to which this condition is departed from by any of the types of system familiar to us, those systems fall short of being perfectly systematic. Reality, then, being a systematic whole, can have no being apart from its appearance, though neither any of them taken singly, nor yet the sum of them thought of collectively,[66] can exhaust its contents. And though no appearance is the whole of Reality, in none of them all does the whole Reality fail to manifest itself as a whole. The whole is truly, as a whole, present in each and every part, while yet no part is the whole.[67] We may once more illustrate by an appeal to our own direct experience. Consider the way in which we set to work to execute any systematic scheme or purpose, _e.g._ the mastery of a particular science or a particular business. We have in such a case a central aim or purpose, which in the process of execution spreads out into a connected system of subordinate ideas and interests welded into one by the reference to a common end which pervades the whole. The supreme or central aim is only realised in the successive realisation of the subordinate stages; at the same time, while it is what sustains all the members of the system, it has no existence apart from them, though it is identical neither with any one of them nor yet with their sum collectively considered.