The mere diversity of our feelings at different moments, it may be said, is of itself incompatible with the strict and absolute _unity_ which is supposed to belong to the thinking principle. If joy and sorrow, such as every one has felt, be different, that which is joyful, and that which is sorrowful, cannot be precisely the same. On the supposition of complete unity and permanence of the thinking principle, nothing is added to it, nothing is taken away from it; and, as it has no parts, no internal change of elementary composition can take place in it. But _that_ to which nothing is added, from which nothing is taken away, and which has no parts to vary their own relative positions and affinities, is so strictly the same, it may be said, that it would surely be absurd to predicate of it any diversity whatever. Joy and sorrow imply an unquestionable diversity of some kind; and if this diversity cannot be predicated of that substance which is precisely the same, without addition, subtraction, or any internal change of composition whatever, that which is joyful, and that which is sorrowful, cannot have absolute identity; or if we affirm, that a diversity, so striking as to form an absolute _contrast_, is yet not inconsistent with complete and permanent unity and identity, we may, in like manner, affirm, that a substance which is hard, heavy, blue, transparent,--which unites with acids, not, with alkalies,--and which is volatilizable at a low temperature,--is precisely the same substance as that, which is soft, light, green, opaque,--which unites with alkalies, not with acids,--and which is absolutely infusible and fixed in the highest temperature to which we can expose it. I have thus endeavoured to place, in the strongest possible light, the most imposing arguments which I can conceive to be urged against the _permanent identity_ of the sentient and thinking principle, that, in combating even _Sophistry_ itself, you may learn, as I have said, to combat with it on equal ground, and assume no advantage but that irresistible advantage which Truth must always afford to him who is the combatant of Error. The positive evidence of the identity of the mind I shall proceed to consider in my next Lecture. FOOTNOTES: [44] Mart. Scrib. chap. vii.--Pope's Works, _edit._ 1757, v. vii. p. 82-84. [45] Essays on the Intellectual Powers, Essay III. chap. iv.--v. 1. p. 341. _Edit. Ed._ 1808. [46] Monsieur Des Cartes. _Shaftesb._ [47] Shaftesbury's Characteristics, vol. iii. p. 172-174. _Edit._ 1745. [48] Pope's Essay on Man, Ep. II. v. 275-282. LECTURE XIII. ON THE DIRECT EVIDENCE OF MENTAL IDENTITY; AND OBJECTIONS ANSWERED.