This fencing, for it does not deserve the name of serious disputation, is rendered possible by the laxness with which the words “identical” and “identity” are commonly used. Bishop Butler would not seriously deny that personality undergoes great changes between infancy and old age, and hence that it must undergo some change from moment to moment. So universally is this recognised, that it is common to hear it said of such and such a man that he is not at all the person he was, or of such and such another that he is twice the man he used to be—expressions than which none nearer the truth can well be found. On the other hand, those whom Bishop Butler is intending to confute would be the first to admit that, though there are many changes between infancy and old age, yet they come about in any one individual under such circumstances as we are all agreed in considering as the factors of personal identity rather than as hindrances thereto—that is to say, there has been no death on the part of the individual between any two phases of his existence, and any one phase has had a permanent though perhaps imperceptible effect upon all succeeding ones. So that no one ever seriously argued in the manner supposed by Bishop Butler, unless with modifications and saving clauses, to which it does not suit his purpose to call attention.
Identical strictly means “one and the same;” and if it were tied down to its strictest usage, it would indeed follow very logically, as we have said already, that no such thing as personal identity is possible, but that the case actually is as Bishop Butler has supposed his opponents without qualification to maintain it. In common use, however, the word “identical” is taken to mean anything so like another that no vital or essential differences can be perceived between them; as in the case of two specimens of the same kind of plant, when we say they are identical in spite of considerable individual differences. So with two impressions of a print from the same plate; so with the plate itself, which is somewhat modified with every impression taken from it. In like manner “identity” is not held to its strict meaning—absolute sameness—but is predicated rightly of a past and present which are now very widely asunder, provided they have been continuously connected by links so small as not to give too sudden a sense of change at any one point; as, for instance, in the case of the Thames at Oxford and Windsor or again at Greenwich, we say the same river flows by all three places, by which we mean that much of the water at Greenwich has come down from Oxford and Windsor in a continuous stream. How sudden a change at any one point, or how great a difference between the two extremes is sufficient to bar identity, is one of the most uncertain things imaginable, and seems to be decided on different grounds in different cases, sometimes very intelligibly, and again at others arbitrarily and capriciously.