_The kind of ideas which can claim truth_
(iii.) “Idea” has two principal meanings.
This distinction is the same as that between our course of ideas and our world of knowledge. We must try now to define it more accurately.
(_a_) An idea as a psychical presentation is strictly a particular. Every moment of consciousness is full of a given complex of presentation which passes away and can never be repeated without some difference. For this purpose a representation is just the same as a presentation; is, in fact, a presentation. Its detail at any given moment is filled in by the influence of the moment, and it can never occur again with precisely the same elements of detail as before. If we use the term “idea” in this sense, as a momentary particular mental state, it is nonsense to speak of having the same idea twice, or of referring it to a reality other than our mental life. The idea in this sense is a psychical image. We cannot illustrate this usage by any recognisable part of our mental furniture, for every such part which can be described and indicated by a general name, is something more than a psychical image. We can only say that that which at any moment we have in consciousness, when our waking perception encounters reality, is such an idea, and so too is the image supplied by memory, when considered simply as a datum, a fact, in our mental history.
(_b_) To get at the other sense of “idea” we should think { 75} of the meaning of a word; a very simple case is that of a proper name. What is the meaning of “St. Paul’s Cathedral in London”? No two people who have seen it have carried away precisely the same image of it in their minds, nor does memory, when it represents the Cathedral to each of them, supply the same image in every detail and association twice over to the same person, nor do we for a moment think that such an image _is_ the Cathedral. [1] Yet we neither doubt that the name _means_ something, and that the same to all those who employ it, nor that it means the same to each of them at one time that it did at every other time. The psychical images which formed the first vision of it are dead and gone for ever, and so, after every occasion on which it has been remembered, are those in which that memory was evoked. The essence of the idea does not lie in the peculiarities of any one of their varying presentations, but in the identical reference that runs through them all, and to which they all serve as material, and the content of this reference _is_ the object of our thought.