When a sea-anemone multiplies by division, there is a time during which it is said to be one animal partially divided; but, after a while, it becomes two animals adherent together, and the limit between these conditions is purely arbitrary. So in mineralogy, a crystal of a definite chemical composition may have its substance replaced, particle by particle, by another chemical compound. When does it lose its primitive identity and become a new thing?
Again, a plant or an animal, in the course of its existence, from the condition of an egg or seed to the end of life, remains the same neither in form, nor in structure, nor in the matter of which it is composed: every attribute it possesses is constantly changing, and yet we say that it is always one and the same individual. And if, in this case, we attribute identity without supposing an indivisible immaterial something to underlie and condition that identity, why should we need the supposition in the case of that succession of changeful phenomena we call the mind?
In fact, we ascribe identity to an individual plant or animal, simply because there has been no moment of time at which we could observe any division of it into parts separated by time or space. Every experience we have of it is as one thing and not as two; and we sum up our experiences in the ascription of identity, although we know quite well that, strictly speaking, it has not been the same for any two moments.
So with the mind. Our perceptions flow in even succession; the impressions of the present moment are inextricably mixed up with the memories of yesterday and the expectations of to-morrow, and all are connected by the links of cause and effect.
" ... as the same individual republic may not only change its members, but also its laws and constitutions; in like manner the same person may vary his character and disposition, as well as his impressions and ideas, without losing his identity. Whatever changes he endures, his several parts are still connected by the relation of causation. And in this view our identity with regard to the passions serves to corroborate that with regard to the imagination, by the making our distant perceptions influence each other, and by giving us a present concern for our past or future pains or pleasures.
"As memory alone acquaints us with the continuance and extent of this succession of perceptions, 'tis to be considered, upon that account chiefly, as the source of personal identity. Had we no memory we never should have any notion of causation, nor consequently of that chain of causes and effects which constitute our self or person. But having once acquired this notion of causation from the memory, we can extend the same chain of causes, and consequently the identity of our persons, beyond our memory, and can comprehend times, and circumstances, and actions, which we have entirely forgot, but suppose in general to have existed. For how few of our past actions are there of which we have any memory? Who can tell me, for instance, what were his thoughts and actions on the first of January, 1715, the eleventh of March, 1719, and the third of August, 1733? Or will he affirm, because he has entirely forgot the incidents of those days, that the present self is not the same person with the self of that time, and by that means overturn all the most established notions of personal identity? In this view, therefore, memory does not so much produce as discover personal identity, by showing us the relation of cause and effect among our different perceptions. 'Twill be incumbent on those who affirm that memory produces entirely our personal identity, to give a reason why we can thus extend our identity beyond our memory.
"The whole of this doctrine leads us to a conclusion which is of great importance in the present affair, viz. that all the nice and subtle questions concerning personal identity can never possibly be decided, and are to be regarded rather as grammatical than as philosophical difficulties. Identity depends on the relations of ideas, and these relations produce identity by means of that easy transition they occasion. But as the relations, and the easiness of the transition may diminish by insensible degrees, we have no just standard by which we can decide any dispute concerning the time when they acquire or lose a title to the name of identity. All the disputes concerning the identity of connected objects are merely verbal, except so far as the relation of parts gives rise to some fiction or imaginary principle of union, as we have already observed.
"What I have said concerning the first origin and uncertainty of our notion of identity, as applied to the human mind may be extended, with little or no variation, to that of simplicity. An object, whose different co-existent parts are bound together by a close relation, operates upon the imagination after much the same manner as one perfectly simple and undivisible, and requires not a much greater stretch of thought in order to its conception. From this similarity of operation we attribute a simplicity to it, and feign a principle of union as the support of this simplicity, and the centre of all the different parts and qualities of the object."—(I. pp. 331-3.)
The final result of Hume's reasoning comes to this: As we use the name of body for the sum of the phenomena which make up our corporeal existence, so we employ the name of soul for the sum of the phenomena which constitute our mental existence; and we have no more reason, in the latter case, than in the former, to suppose that there is anything beyond the phenomena which answers to the name. In the case of the soul, as in that of the body, the idea of substance is a mere fiction of the imagination. This conclusion is nothing but a rigorous application of Berkeley's reasoning concerning matter to mind, and it is fully adopted by Kant.[35]
Having arrived at the conclusion that the conception of a soul, as a substantive thing, is a mere figment of the imagination; and that, whether it exists or not, we can by no possibility know anything about it, the inquiry as to the durability of the soul may seem superfluous.
Nevertheless, there is still a sense in which, even under these conditions, such an inquiry is justifiable. Leaving aside the problem of the substance of the soul, and taking the word "soul" simply as a name for the series of mental phenomena which make up an individual mind; it remains open to us to ask, whether that series commenced with, or before, the series of phenomena which constitute the corresponding individual body; and whether it terminates with the end of the corporeal series, or goes on after the existence of the body has ended. And, in both cases, there arises the further question, whether the excess of duration of the mental series over that of the body, is finite or infinite.
Hume has discussed some of these questions in the remarkable essay On the Immortality of the Soul, which was not published till after his death, and which seems long to have remained but little known. Nevertheless, indeed, possibly, for that reason, its influence has been manifested in unexpected quarters, and its main arguments have been adduced by archiepiscopal and episcopal authority in evidence of the value of revelation. Dr. Whately,[36] sometime Archbishop of Dublin, paraphrases Hume, though he forgets to cite him; and Bishop Courtenay's elaborate work,[37] dedicated to the Archbishop, is a development of that prelate's version of Hume's essay.
This little paper occupies only some ten pages, but it is not wonderful that it attracted an acute logician like Whately, for it is a model of clear and vigorous statement. The argument hardly admits of condensation, so that I must let Hume speak for himself:—