In Nature it is the potent action which is real. It alone can be directly represented by the activity of Thought. The mere obstruction of activity is not a real thing, hence the unreal character of Sensation. Yet the obstruction being an obstruction of the real action of Nature is, if not real, at least actual and immediate. Nay, its presence in our Experience, however mutable and unstable it may be, is the only sure test and guarantee of Reality. Each of the two leading theories which have dominated speculation presents one partial aspect of the truth. The eternal cognisable element of Reality _is_ apprehended, as the Platonist holds, by the intellect and by the intellect alone. To that extent the Platonist is right. That cognisable element is Action. But Action is denoted for us only in the obstructions which it encounters. These obstructions constitute our World of Sensible Experience, which is therefore for each of us the sure indicator of the Real. In recognising this fact the sensationalist is right in his turn. * * * * * Not only does the dynamic conception of Nature enable us to account for Sensation, but it lets us see how the Sensible World becomes a constituent of Experience. It is by and through its obstructions and these only that we featurise or denote our Experience. It is by the breaks, the turnings in the road that we cognise its course. It is by the line of rocks and breakers that we define the shore. But we must not mistake the turnings for the roadway nor the shore for the ocean. It is in and by our activity that we discover this World of sensible obstructions. The features of the Sensible World correspond therefore to the laws of our exertional activity, but the correspondence is relational, not resemblant. Just so, it is by the reflection of Light that we discover the forms of the obstacle which solid bodies oppose to the radiant undulation. The resultant colours correspond to the form of these obstructions; but the correspondence is relational not resemblant. The same is true of sounds, of tactual sensations, of every other sensible obstacle to pure activity. By the clouds of smoke we follow or used to follow the progress of the battle, but the battle is something other than a cloud of smoke. We are, as Plato told us in his famous allegory, like prisoners in a cave--our attitude averted from the aperture, and it is only by the shadows cast upon the cavern wall that we can interpret the events which are transacting themselves outside. In one sense, therefore, the whole sensible and spatial World is real. At least it is actual; and it affords us the materials from which we construct our scheme of phenomena, and by which the kinetic process of Reality is denoted and conceived.