200 G. S. FULLERTON : merely a concrete instance of this general truth which all must either explicitly or implicitly admit. As I have said, the elements constituent of this conception are the usual qualitative attributes of a line and the notion of continued progression, of unlimited possibility of quantity. These elements may be brought into mind, to the exclusion of the notion of limits, which are yet present in all imagined lines and in all intuitions of lines in nature, by employing the usual process of forming a concept. When I think of an infinite line, I first represent to myself a line of some inde- finite length, and I then run mentally along this line, adding new portions ; that is, I successively think several increasing lengths : I have now before my mind what Hume and Prof. Bain insist upon and make so prominent in forming the concept, several concrete objects similar in some of their qualities. Having mentally passed over several of these line-portions, I then fix my attention, not upon that in which they differ, the quantitative element, but upon that in which they agree, the usual qualitative attributes of a line, and the element of increase or progression, which is common to all. This is precisely what I do in forming the concept man or animal. The concrete objects are compared, their differences eliminated by abstraction, and their likenesses grasped together under a distinctive name. Or I may select one of the qualities in which objects agree and consider it alone, as when I compare men, of the same age and colour, only as to their height, and pronounce them equal in height. If this be possible, if, in using the word man, I can distin- guish between that in which men agree and that in which they disagree, and if it be further possible for me to fix my attention upon one of the points in which they agree to the exclusion of others ; then it is possible to abstract from the particular quantities or amounts of several lines present in imagination, and think only of a constant increase or pro- gression. That both the one and the other are not only possible but actual operations, is proved beyond possibility of doubt by our constant comparison of objects, our use of general language, our frequent use of the word 'infinite,' to indicate what is clearly distinguished, readily defined, and conveys a distinct meaning to speaker and hearer. 1 1 It might be objected that the case cited is not fairly analogous, on the ground that an end' is not an actual constituent element of an object, which may exist and be contemplated apart from the other elements. But the s-n.sitioii of progression in passing the eye ahmj,' a line, is very clearly ditli rent fi-'im the sensation of arrested motion, of an end to the intuition, and there is no reason why these two sensations should not be separated by
