I subjoin a passage from the work of Professor Bain on Psychology, where this difficult subject is carefully analysed (The Senses and the Intellect, p. 370). "There is no possible knowledge of the world except in reference to our minds. Knowledge means a state of mind: the knowledge of material things is a mental thing. We are incapable of discussing the existence of an independent material world: the very act is a contradiction. We can speak only of a world presented to our own minds. By an illusion of language we fancy that we are capable of contemplating a world which does not enter into our own mental existence: but the attempt belies itself, for this contemplation is an effort of mind." "Solidity, extension, space--the foundation properties of the material world--mean, as has been said above, certain movements and energies of our own bodies, and exist in our minds in the shape of feelings of force, allied with visible and tactile, and other sensible impressions. The sense of the external is the consciousness of particular energies and activities of our own." (P. 376). "We seem to have no better way of assuring ourselves and all mankind, that with the conscious movement of opening the eyes there will always be a consciousness of light, than by saying that the light exists as an independent fact, without any eyes to see it. But if we consider the fact fairly we shall see that this assertion errs, not simply in being beyond any evidence that we can have, but also in being a self-contradiction. We are affirming _that_ to have an existence out of our minds, which we cannot know but as in our minds. In words we assert independent existence, while in the very act of doing so we contradict ourselves. Even a possible world implies a possible mind to conceive it, just as much as an actual world implies an actual mind. The mistake of the common modes of expression on this matter is the mistake of supposing the abstractions of the mind to have a separate and independent existence. Instead of looking upon the doctrine of an external and independent world as a generalisation or abstraction grounded on our particular experiences, summing up the past and predicting the future, we have got into the way of maintaining the abstraction to be an independent reality, the foundation, or cause, or origin, of all these experiences." To the same purpose Mr. Mansel remarks in his Bampton Lectures on "The Limits of Religious Thought," page 52: