This difficulty may appear at first sight purely verbal, and we shall perhaps be inclined to see the answer to it in the double use that we make of the word knowledge. We use the word in two senses, in one of which it includes all and everything that is or can be present to the mind in thinking, and in another and narrower sense the word knowledge means truth. It was in the narrow sense of the word that whatever is not true is not knowledge that Socrates interpreted the meaning of the Delphic oracle that had declared him the wisest of men. His wisdom must be, he said, that whereas other men seemed to be wise and to know something, he knew that he knew nothing. All men have opinion, but opinion is not knowledge, though easily and generally mistaken for it. His perplexity was to understand what actually this false opinion could be which passed for knowledge. It could not be nothing at all, for then it would simply mean ignorance; but in false opinion {76} some object is present to the mind. Everything that the mind thinks of has being. A thing may have being that does not exist if by existence is meant the particular existence of an event in time, for most of the things we think about are timeless--they are ideas, such as whiteness, goodness, numbers and the properties of numbers, faith, love, and such-like. All such ideas are called universals, because their reality does not mean that they exist at one particular moment and no other, but they are real, they have being. How, then, can there be anything intermediate between being and not being, anything that is and also is not, for this is what false opinion or error seems to be? There is, then, a problem of error, and it is quite distinct from the problem of truth. The problem of truth is to know by what criterion we can test the agreement of our ideas with reality; the problem of error is to know how there can be false opinion. There is false opinion, of this no one needs to be convinced; but where its place is in the fundamental scheme of the mental process, in what precisely it consists, whether it is purely a negation or whether it has a positive nature of its own, this is the problem we have now to consider.