The periodic theory might be held in forms in which this uncanny doctrine of absolute identity was avoided; but at the best it meant an endless monotonous iteration, which was singularly unlikely to stimulate speculative interest in the future. It must be remembered that no thinker had any means of knowing how near to the end of his cycle the present hour might be. The most influential school of the later Greek age, the Stoics, adopted the theory of cycles, and the natural psychological effect of the theory is vividly reflected in Marcus Aurelius, who frequently dwells on it in his Meditations. "The rational soul," he says, "wanders round the whole world and through the encompassing void, and gazes into infinite time, and considers the periodic destructions and rebirths of the universe, and reflects that our posterity will see nothing new, and that our ancestors saw nothing greater than we have seen. A man of forty years, possessing the most moderate intelligence, may be said to have seen all that is past and all that is to come; so uniform is the world." [Footnote: xi. I. The cyclical theory was curiously revived in the nineteenth; century by Nietzsche, and it is interesting to note his avowal that it took him a long time to overcome the feeling of pessimism which the doctrine inspired.] 3. And yet one Stoic philosopher saw clearly, and declared emphatically, that increases in knowledge must be expected in the future. "There are many peoples to-day," Seneca wrote, "who are ignorant of the cause of eclipses of the moon, and it has only recently been demonstrated among ourselves. The day will come when time and human diligence will clear up problems which are now obscure. We divide the few years of our lives unequally between study and vice, and it will therefore be the work of many generations to explain such phenomena as comets. One day our posterity will marvel at our ignorance of causes so clear to them. "How many new animals have we first come to know in the present age? In time to come men will know much that is unknown to us. Many discoveries are reserved for future ages, when our memory will have faded from men's minds. We imagine ourselves initiated in the secrets of nature; we are standing on the threshold of her temple." [Footnote: The quotations from Seneca will be found in Naturales Quaestiones, vii. 25 and 31. See also Epist. 64. Seneca implies continuity in scientific research. Aristotle had stated this expressly, pointing out that we are indebted not only to the author of the philosophical theory which we accept as true, but also to the predecessors whose views it has superseded (Metaphysics, i. ii. chap. 1). But he seems to consider his own system as final.]