This fact will be more clearly recognised, perhaps, if we remember that the cogency of our ideal knowledge follows upon our intent in fixing its object. It hangs on a virtual definition, and explicates it. We cannot oblige anybody or anything to reproduce the idea which we have chosen; but that idea will remain the idea it is whether forgotten or remembered, exemplified or not exemplified in things. To penetrate to the foundation of being is possible for us only because the foundation of being is distinguishable quality; were there no set of differing characteristics, one or more of which an existing thing might appropriate, existence would be altogether impossible. The realm of essence is merely the system or chaos of these fundamental possibilities, the catalogue of all exemplifiable natures; so that any experience whatsoever must tap the realm of essence, and throw the light of attention on one of its constituent forms. This is, if you will, a trivial achievement; what would be really a surprising feat, and hardly to be credited, would be that the human mind should grasp the _constitution of nature_; that is, should discover which is the particular essence, or the particular system of essences, which actual existence illustrates. In the matter of physics, truly, we are reduced to skimming the surface, since we have to start from our casual experiences, which form the most superficial stratum of nature, and the most unstable. Yet these casual experiences, while they leave us so much in the dark as to their natural basis and environment, necessarily reveal each its ideal object, its specific essence; and we need only arrest our attention upon it, and define it to ourselves, for an eternal possibility, and some of its intrinsic characters, to have been revealed to our thought.