[58] But that this is a mere fiction of fable it is easy to see from many circumstances. For the very moment that the first man was born there was a necessity for his receiving growth in accordance with the previously defined measures and numbers of time, for nature has arranged the different ages as certain steps along which man in a manner ascends and descends; he ascends while he is growing, and he descends at the period when he is lessening; and the boundary of the uppermost steps is the prime of life at which when a man has arrived he no longer makes any further advance; but as runners who run the diaulos turn back again upon the same path which they have already travelled, so too does man retrace his steps, giving back in the weakness of old age what he has received from vigorous youth;