Concept v. image. At this point, however, it is most important to expose an ambiguity in the word "idea" of which the philosophers we have just been discussing seem to have been unaware: In using the word "idea" one can be talking about either a concept or an image. Most of the time, they appear to have been talking about images, but sometimes the discussion of "ideas'' shifted in such a way that it would be more appropriate to a discussion of concepts. Without having seen red we cannot form in our minds red images; but from this we cannot conclude that we can have no concept of red. To illustrate this point, let us take the example of ultraviolet. No human being can have ultraviolet images, since the human eye is not sensitive to that part of the spectrum; bees and certain other creatures can see it, but we cannot Since we have no ultraviolet impressions, we can have no ultraviolet images. But we do appear to have a concept of ultraviolet. Physicists speak of ultraviolet light, and can identify it and relate it to other parts of the spectrum; indeed, they can talk about ultraviolet just as easily as they talk about red. Similarly, human beings do not have any sense that acquaints them with the presence of radioactivity the way they have senses like sight and hearing and touch that acquaint them with the sensible properties of physical objects. ("Sensible" in philosophy means "capable of being sensed.") We can't see, hear, smell, or touch radioactivity; we have to rely on instruments like Geiger counters to detect its presence. If any creature did have a sense acquainting him directly with the presence of radioactivity, we would have not the faintest conception of what it would be like; we simply have no "image" of radioactivity. (Remember that images need not be visual: there are auditory images, tactile images, olfactory images, and so on. When you imagine the smell of ammonia or the taste of scalloped potatoes, you are having olfactory images and gustatory images respectively.) Yet we do, it seems, have the concepts at any rate physicists do and physicists work as easily and familiarly with this concept as they do with concepts of which they do have sense-impressions (and consequently images). Hume's dictum "If no impressions, then no ideas" applies to images; it does not seem to apply to images; it does not seem to apply to concepts.