Nay so entire is the custom, that the very same idea may be annexed to several different words, and may be employed in different reasonings, without any danger of mistake. Thus the idea of an equilateral triangle of an inch perpendicular may serve us in talking of a figure, of a rectilineal figure, of a regular figure, of a triangle, and of an equilateral triangle. All these terms, therefore, are in this case attended with the same idea; but as they are wont to be applied in a greater or lesser compass, they excite their particular habits, and thereby keep the mind in a readiness to observe, that no conclusion be formed contrary to any ideas, which are usually comprised under them. Before those habits have become entirely perfect, perhaps the mind may not be content with forming the idea of only one individual, but may run over several, in order to make itself comprehend its own meaning, and the compass of that collection, which it intends to express by the general term. That we may fix the meaning of the word, figure, we may revolve in our mind the ideas of circles, squares, parallelograms, triangles of different sizes and proportions, and may not rest on one image or idea. However this may be, 'tis certain _that_ we form the idea of individuals whenever we use any general term; _that_ we seldom or never can exhaust these individuals; and _that_ those which remain, are only represented by means of that habit by which we recal them, whenever any present occasion requires it. This then is the nature of our abstract ideas and general terms; and 'tis after this manner we account for the foregoing paradox, _that some ideas are particular in their nature, but general in their representation_. A particular idea becomes general by being annexed to a general term; that is, to a term which, from a customary conjunction, has a relation to many other particular ideas, and readily recals them in the imagination. The only difficulty that can remain on this subject, must be with regard to that custom, which so readily recals every particular idea for which we may have occasion, and is excited by any word or sound to which we commonly annex it. The most proper method, in my opinion, of giving a satisfactory explication of this act of the mind, is by producing other instances which are analogous to it, and other principles which facilitate its operation. To explain the ultimate causes of our mental actions is impossible. 'Tis sufficient if we can give any satisfactory account of them from experience and analogy. First, then, I observe, that when we mention any great number, such as a thousand, the mind has generally no adequate idea of it, but only a power of producing such an idea, by its adequate idea of the decimals under which the number is comprehended. This imperfection, however, in our ideas, is never felt in our reasonings, which seems to be an instance parallel to the present one of universal ideas.