Consciousness is in constant change. We do not mean by this to say that no one state of mind has any duration -- even if true, that would be hard to establish.

What we wish to lay stress on is this, that no state once gone can recur and be identical with what it was before.

Now we are seeing, now hearing; now reasoning, now willing; now recollecting, now expecting; now loving, now hating; and in a hundred other ways we know our minds to be alternately engaged.

The grass out of the window now looks to us of the same green in the sun as in the shade, and yet a painter would have to paint one part of it dark brown, another part bright yellow, to give its real sensational effect.

We take no heed, as a rule, of the different way in which the same things look and sound and smell at different distances and under different circumstances.

The sameness of the things is what we are concerned to ascertain; and any sensations that assure us of that will probably be considered in a rough way to be the same with each other.