In training-institutions for the blind they teach the pupils as much _about_ light as in ordinary schools. Reflection, refraction, the spectrum, the ether-theory, etc., are all studied. But the best taught born-blind pupil of such an establishment yet lacks a knowledge which the least instructed seeing baby has. They can never show him what light is in its 'first intention'; and the loss of that sensible knowledge no book-learning can replace. All this is so obvious that we usually find sensation 'postulated' as an element of experience, even by those philosophers who are least inclined to make much of its importance, or to pay respect to the knowledge which it brings.[4] But the trouble is that most, if not all, of those who admit it, admit it as a fractional _part_ of the thought, in the old-fashioned atomistic sense which we have so often criticised. Take the pain called toothache for example. Again and again we feel it and greet it as the same real item in the universe. We must therefore, it is supposed, have a distinct pocket for it in our mind into which it and nothing else will fit. This pocket, when filled, is the sensation of toothache; and must be either filled or half-filled whenever and under whatever form toothache is present to our thought, and whether much or little of the rest of the mind be filled at the same time. Thereupon of course comes up the paradox and mystery: If the knowledge of toothache be pent up in this separate mental pocket, how can it be known _cum alio_ or brought into one view with anything else? This pocket knows nothing else; no other part of the mind knows toothache. The knowing of toothache _cum alio_ must be a miracle. And the miracle must have an Agent. And the Agent must be a Subject or Ego 'out of time,'--and all the rest of it, as we saw in Chapter X. And then begins the well-worn round of recrimination between the sensationalists and the spiritualists, from which we are saved by our determination from the outset to accept the psychological point of view, and to admit knowledge whether of simple toothaches or of philosophic systems as an ultimate fact. There are realities and there are 'states of mind,' and the latter know the former; and it is just as wonderful for a state of mind to be a 'sensation' and know a simple pain as for it to be a thought and know a system of related things.[5] But there is no reason to suppose that when different states of mind know different things about the same toothache, they do so by virtue of their all _containing_ faintly or vividly the original pain. Quite the reverse. The by-gone sensation of my gout was painful, as Reid somewhere says; the _thought_ of the same gout as by-gone is pleasant, and in no respect resembles the earlier mental state.