Logicians appear to have taken little notice of a class of terms which partake in certain respects of the character of abstract terms and yet are undoubtedly the names of concrete existing things. These terms are the names of substances, such as gold, carbonate of lime, nitrogen, &c. We cannot speak of two golds, twenty carbonates of lime, or a hundred nitrogens. There is no such distinction between the parts of a uniform substance as will allow of a discrimination of numerous individuals. The qualities of colour, lustre, malleability, density, &c., by which we recognise gold, extend through its substance irrespective of particular size or shape. So far as a substance is gold, it is one and the same everywhere; so that terms of this kind, which I propose to call *substantial terms*, possess the peculiar unity of abstract terms. Yet they are not abstract; for gold is of course a tangible visible body, entirely concrete, and existing independently of other bodies. It is only when, by actual mechanical division, we break up the uniform whole which forms the meaning of a substantial term, that we introduce number. *Piece of gold* is a term capable of plurality; for there may be a great many pieces discriminated either by their various shapes and sizes, or, in the absence of such marks, by simultaneously occupying different parts of space. In substance they are one; as regards the properties of space they are many.[45] We need not further pursue this question, which involves the distinction between unity and plurality, until we consider the principles of number in a subsequent chapter. [45] Professor Robertson has criticised my introduction of “Substantial Terms” (*Mind*, vol. i. p. 210), and objects, perhaps correctly, that the distinction if valid is extra-logical. I am inclined to think, however, that the doctrine of terms is, strictly speaking, for the most part extra-logical. *Collective Terms.* We must clearly distinguish between the *collective* and the *general meanings* of terms. The same name may be used to denote the whole body of existing objects of a certain kind, or any one of those objects taken separately. “Man” may mean the aggregate of existing men, which we sometimes describe as *mankind*; it is also the general name applying to any man. The vegetable kingdom is the name of the whole aggregate of *plants*, but “plant” itself is a general name applying to any one or other plant. Every material object may be conceived as divisible into parts, and is therefore collective as regards those parts. The animal body is made up of cells and fibres, a crystal of molecules; wherever physical division, or as it has been called *partition*, is possible, there we deal in reality with a collective whole. Thus the greater number of general terms are at the same time collective as regards each individual whole which they denote.