What is a concept? A concept is to be distinguished from an intuitive image above all by the fact that it is completely determined and has nothing uncertain about it. One might be tempted to say — and many logicians have indeed said — that a concept is simply an image with a strictly fixed content. As we have seen, however, there are no such entities in psychological reality because all images are to one degree or another vague. One might of course suppose that images with fixed content are at least possible; but this supposition would still be limited to individual images. It would not apply at all in the case of general ideas or images, and these are what we need for knowing; for, as we have just made clear, general images cannot possibly exist as real mental entities.