Page:Treatise of Human Nature (1888).djvu/41

 raises up an individual idea, along with a certain custom; and that custom produces any other individual one, for which we may have occasion. But as the production of all the ideas, to which the name may be apply’d, is in most cases impossible, we abridge that work by a more partial consideration, and find but few inconveniences to arise in our reasoning from that abridgment.

For this is one of the most extraordinary circumstances in the present affair, that after the mind has produc’d an individual idea, upon which we reason, the attendant custom, reviv’d by the general or abstract term, readily suggests any other individual, if by chance we form any reasoning, that agrees not with it. Thus shou’d we mention the word, triangle, and form the idea of a particular equilateral one to correspond to it, and shou’d we afterwards assert, that the three angles of a triangle are equal to each other, the other individuals of a scalenum and isoceles, which we overlook’d at first, immediately crowd in upon us, and make us perceive the falshood of this proposition, tho’ it be true with relation to that idea, which we had form’d. If the mind suggests not always these ideas upon occasion, it proceeds from some imperfection in its faculties; and such a one as is often the source of false reasoning and sophistry. But this is principally the case with those ideas which are abstruse and compounded. On other occasions the custom is more entire, and ’tis seldom we run into such errors.

Nay so entire is the custom, that the very same idea may be annext to several different words, and may be employ’d 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 apply’d in a greater or lesser compass, they excite their particular habits, and thereby keep the mind in a readiness to observe, that no