the problem has been driven into narrower and narrower limits, until at last we are left with the _infimæ species_ and the individuals contained under them. It remains to discover in what relation these stand to one another. The answer is unsatisfactory. We are told that there is no definition of individuals, and also that the definition is identical with the individual.[244] Such, indeed, is the conclusion necessarily resulting from Aristotle’s repeated declarations that all knowledge is of definitions, that all knowledge is of something really existing, and that nothing really exists but individual things. Nevertheless, against these we have to set equally strong declarations to the effect that knowledge is of something general, not of the perishing individuals which may pass out of existence at any moment. The truth is, that we are here, as Zeller has shown,[245] in presence of an insoluble contradiction, and we must try to explain, not how Aristotle reconciled it with itself, for that was impossible, but how he reconciled himself to it. His analysis of individuality was the first step in this direction. We have seen that he treats definition as a process of gradual specification, beginning with the most general notions, and working down by successive differentiations to the most particular. Now, the completed conception is itself the integration of all these differences, the bond of union holding them together. Turning to an antithetical order of ideas, to the material substance of which bodies are composed, and its various transformations, we find him working out the same vein of thought. According to the Aristotelian chemistry, an ultimate indeterminate unknowable something clothes itself with one or other of the opposing attributes, dry and moist, hot and cold; and when two of these are combined, manifests itself to our senses as one of the four elements. The elements combine in a particular manner to form homogeneous animal tissues, and these again are united into heterogeneous organs, which together constitute the living body. Here, then, we have two analogous series of specifications—one conceptual and leading down from the abstract to the concrete, the other physical, and leading up from the vague, the simple, and the homogeneous, to the definite, the complex, and the heterogeneous. Aristotle embraces both processes under a single comprehensive generalisation. He describes each of them as the continuous conversion of a possibility into an actuality. For the sake of greater clearness, let us take the liberty of substituting modern scientific terms for his cumbrous and obsolete classifications. We shall then say that the general notion, living thing, contains under it the two less general notions—plant and animal. If we only know of any given object that it has life, there is implied the possibility of its being either the one or the other, but not both together. On determining it to be (say) an animal, we