Death, we might say, lies heaviest on the living. The dead, as Socrates wisely observes (Apologia), are either wholly unconscious, or sufficiently happy not to regret their deaths; and so, it is the living who must suffer perhaps the greatest sorrow of all, and the chief and source and type of all sorrows: the separation from that which we love.
That such separation is the type and source of all sorrow, is easily demonstrated. The death of a friend or relative; the loss of health, wealth, or status; the failure of some plan; the break-down of a marriage or a family; the bitter end of a friendship; loss of a livelihood; denial of a reward; exile from a place dear to one’s heart; a child’s cry at removal from its parents, or or even from a favourite toy, are all forms of separation from something beloved, and inspire sorrow for precisely that reason.
That we all die, and nothing is imperishable, is an accepted fact in practice, though seldom taught to children. That we are reborn is less certain, though it seems required by the principle of conservation of energy, and consistent with the recycling process common in nature. If no energy is truly annihilated, except under such (relatively) rare conditions as the demise of the universe itself, and at the event horizons of black holes, the substances whereof we are made may be said to exist for ever, and indeed to have existed since the Big Bang. Moreover, if matter in the short term is recycled in the ordinary course of nature (as is not infrequently the case), we are indeed reborn, though in a manner far more modest than our ancestors imagined.
But we do not remember our past lives. Those who claim to do so, are seldom reliable; and those who reliably do so (that is, whose accounts of a past life can be proven, and yet found to come from no other source) are few and far between. There may be sound evolutionary reasons for this forgetfulness. The absence, from nearly all vertebrates, of a past-life memory, suggests the nervous system of vertebrates is not equipped to retain these memories; presumably because, if it did so, it would confer no selective advantage whatsoever. What advantages one may have of lessons learnt in a previous existence, says the usual doctrine of re-incarnation, appear of themselves and unconsciously, without the need of either memory or volition, and are neither helped nor hindered by either.
As rather a moral theory (its usual function) than an ontological, this doctrine, in all its appearances (Vaishnavism, Jainism, Buddhism, Pythagoreanism, Socratic thought, and Theosophy, among others), advises us to live honorably in the present life and at the present moment, and the let the result thereof decide our future. Remembrance, if any, is a reward rather than the means of this, and the especial privilege of prophets. The doctrine of Heaven and Hell, found in Zoroastrian, Judaic, Christian, Muslim, and similar traditions, says much the same, though it ignores the possibility of a rebirth in this world, after paying (and receiving) one’s dues in the other; for in these traditions, it is only the prophets who see Heaven and Hell, while the multitude must take their word and have faith in it.
It may console us, meanwhile, to remember that death is often poetically likened to sleep; and if this likeness prove true in more than the poetic sense, it raises a few comforting possibilities of its own.
Most people do not remember what passes in their sleep, save for fragments of particular dreams; and this pittance of a remembrance becomes less and less frequent, though the dreams and other arousals do not, as one grows older. Therefore it makes an intuitive, if not quite a scientific sense, that we no more remember a past life, or the interval (if any) between lives, than we remember the moment we fall asleep, or the majority of dreams and other partial wakings sleep is heir to. Sleep is often broken by nervous spasms, and sleepers are known to talk, walk, and carry out daily routines in their sleep; or to toss and turn, glance wildly about, pass in and out of amorous states, &c., and remember none of it when woken. Death, if it does not entail the entire annihilation of the psyche, may be beset with similar interruptions (as most mythology about it would suggest), which all pass unnoticed.