The chief function of wise and older men toward their juniors, they thought, was to prevent the premature hardening of opinions into convictions and to keep their minds open and growing. As we now often say that the chief function of religion and sex is to keep each other pure, so they thought that wisdom culminated in eros, which in turn found its highest deployment in the love of knowledge, which Aristotle later described as the theoretic life, the attainment of which he deemed the supreme felicity of man. From all this it follows that those who achieve complete ideal senescence are those who have entirely sublimated eroticism into the passion for truth and pursue it with the same ardor that in their prime attracted them to the most beautiful of the other sex; and that their chief function to the next generation is to lay in it the foundations for the same gradual transfer and transformation of it as old age advances. Aristotle’s[55] physical theory of old age is that heat is lost by gradual dissipation, very little remaining in old age--a flickering flame that a slight disturbance could put out. The lung hardens by gradual evaporation of the fluid and so is unable to perform its office of heat regulation. He assumes that heat is gradually developed in the heart. The amount produced is always somewhat less than that which is given off and the deficiency has to be made good out of the stock with which the organism started originally, that is, from the innate heat in which the soul was incorporate. This eventually is so reduced by constant draughts made upon it that it is insufficient to support the soul. The natural span of life, he says, differs greatly in length in different species, due to material constitution and the degree of harmony with the environment. But still, as a general rule, big plants and animals live longer than smaller ones; sanguineous or vertebrates longer than invertebrates; the more perfect longer than the less perfect; and long gestation generally goes with long duration. Thus bulk, degree of organization, period of gestation, are correlated. Great size goes with high organization.