To identity, it may be said, it is necessary that the _qualities_ be the same. That of which the qualities are different, cannot be the same; and the only mode of discovering whether a substance have the same or different qualities, is to observe, how it affects and is affected by other substances. It is recognized by us as the same, or, at least, as perfectly similar, when, in two corresponding series of changes, the same substances affect _it_ in the same manner, and _it_ affects, in the same manner, the same substances; and when either the same substances do not affect it in the same manner, or it does not affect, in the same manner, the same substances, we have no hesitation in considering it as _different_. Thus, if a white substance, resembling exactly, in every external appearance, a lump of sugar, do not _melt_ when exposed to the action of boiling water, we do not regard it as _sugar_, because the water does not act on it as we have uniformly known it to act on that substance; or if the same white lump, in every other respect resembling sugar, affect our taste as _bitter_ or acrid rather than sweet, we immediately, in like manner, cease to consider it as sugar, because it does not act upon our nerves of taste in the same manner as sugar acts upon them. The complete similarity, in other respects, is far from sufficient to make us alter our judgment; a single circumstance of manifest difference, in its mode either of acting upon other substances, or of being acted upon by them, being sufficient to destroy the effect of a thousand manifest resemblances. Let this test of identity, then, it may be said, be applied to the _mind_, at different periods, if the test be allowed to be a just one; and let it be seen, whether, in the series of changes in which it acts or is acted upon, the phenomena precisely correspond in every case. If the same objects do _not_ act upon it in the same manner, it must then be _different_, according to the very definition to which we are supposed to have assented.--You, of course, understand, that I am at present only assuming the character of an objector, and that I state an argument, the principle of which you will afterwards find to be false.