But the feeling of resemblances is one thing and the idea of class is another. The class is the external framework of which objects perceived to be similar form, in part, the contents. Now the contents cannot furnish the frame into which they fit. They are made up of _vague and fluctuating_ images, due to the super-imposition and partial fusion of a _determined number of individual images_, which are found to have common elements; the framework, on the contrary, is a _definite form_, with fixed outlines, but which may be applied to an _undetermined number of things_, perceived or not, actual or possible. In fact, every class has possibilities of extension which go far beyond the circle of objects which we know, either from direct experience or from resemblance. This is why every school of thinkers has refused, and not with good reason, to identify the idea of class with that of a generic image. The generic image is only the indistinctly-bounded residual representation left in us by similar representations, when they are present in consciousness simultaneously; the class is a logical symbol by means of which we think distinctly of these similarities and of other analogous ones. Moreover, the best proof of the distance separating these two notions is that an animal is able to form generic images though ignorant of the art of thinking in classes and species. The idea of class is an instrument of thought which has obviously been constructed by men. But in constructing it, we have at least had need of a model; for how could this idea ever have been born, if there had been nothing either in us or around us which was capable of suggesting it to us? To reply that it was given to us _a priori_ is not to reply at all; this lazy man's solution is, as has been said, the death of analysis. But it is hard to see where we could have found this indispensable model except in the spectacle of the collective life. In fact, a class is not an ideal, but a clearly defined group of things between which internal relationships exist, similar to those of kindred. Now the only groups of this sort known from experience are those formed by men in associating themselves. Material things may be able to form collections of units, or heaps, or mechanical assemblages with no internal unity, but not groups in the sense we have given the word. A heap of sand or a pile of rock is in no way comparable to that variety of definite and organized society which forms a class. In all probability, we would never have thought of uniting the beings of the universe into homogeneous groups, called classes, if we had not had the example of human societies before our eyes, if we had not even commenced by making things themselves members of men's society, and also if human groups and logical groups had not been confused at first.[452]