It is also claimed by some thinkers that the idea of future-existence presupposes past-existence, for everything that is "begun" must "end" some time, and therefore if we are to suppose that the soul is to continue its existence in the future, we must think of it as having an existence in the past--being eternal at both ends of the earth-life, as it were. Opponents of the idea of immortality are fond of arguing that there was no more reason for supposing that a soul would continue to exist after the death of the body, than there was for supposing that it had existed previously. A well-known man once was asked the question: "What becomes of a man's soul after death?" when he evaded the question by answering: "It goes back to where it came from." And to many this idea has seemed sufficient to make them doubt the idea of immortality. The ancient Greek philosophers felt it logically necessary for them to assert the eternal pre-existence of the soul in order to justify their claim of future existence for it. They argued that if the soul is immortal, it must have always existed, for an immortal thing could not have been created--if it was not immortal by nature, it could never be made so, and if it was immortal by nature, then it had always existed. The argument usually employed is this: A thing is either mortal or immortal, one or the other; if it is mortal it has been born and must die; if it is immortal, it cannot have been born, neither can it die; mortality means subject to life and death--immortality means immunity from both. The Greeks devoted much time and care to this argument, and attached great importance to it. They reasoned that nothing that possessed Reality could have emerged from nothingness, nor could it pass into nothingness. If it were Real it was Eternal; if it was not Eternal it was not Real, and would pass away even as it was born. They also claimed that the sense of immortality possessed by the Ego, was an indication of its having experienced life in the past, as well as anticipating life in the future--there is a sense of "oldness" pervading every thought of the soul regarding its own nature. It is claimed as an illogical assumption to hold that back of the present there extends an eternity of non-existence for the soul, while ahead of it there extends an eternity of being--it is held that it is far more logical to regard the present life as merely a single point in an eternity of existence.