When Sri Aurobindo describes the three stages of human evolution, he does not imply that these are clearly separated and demarcated phases which take place sequentially in a direct and clear manner. Rather, these phases are part of a highly complex and somewhat confusing interaction and mixture such that all three are at least partly in evidence in varying degrees throughout human history and human society. What is being described is a broad-brush view of the general stage that characterizes an age of human life and society, while recognizing the admixture that actually is taking place in all beings as the divine being slowly emerges from the infra-rational animal stage through the more or less mental-development phase of existence. The interaction of these three phases corresponds more or less to the action of the three gunas, or qualities, described in the ancient texts, — tamas, rajas, sattwa, which interplay to create the entire universal creation and exert their influence by predominant action with whatever admixture of the other two, through a constantly changing and evolving balance.
Sri Aurobindo explains: “..we must not suppose that they are naturally exclusive and absolute in their nature, or complete in their tendency or fulfilment when they come, or rigidly marked off from each other in their action or their time. For they not only arise out of each other, but may be partially developed in each other and they may come to coexist in different parts of the earth at the same time. But, especially, since man as a whole is always a complex being, even man savage or degenerate, he cannot be any of these things exclusively or absolutely, — so long as he has not exceeded himself, has not developed into the superman, has not, that is to say spiritualised and divinised his whole being. At his animal worst he is still some kind of thinking or reflecting animal: even the infrarational man cannot be utterly infrarational, but must have or tend to have some kind of play more or less evolved or involved of the reason and a more or less crude suprarational element, a more or less disguised working of the spirit. At his lucid mental best, he is still not a pure mental being, a pure intelligence; even the most perfect intellectual is not and cannot be wholly or merely rational, — there are vital urgings that he cannot exclude, visits or touches of a light from above that are not less suprarational because he does not recognise their source. No god, but at his highest a human being touched with a ray of the divine influence, man’s very spirituality, however dominant, must have, while he is still this imperfectly evolved human, its rational and infrarational tendencies and elements. And as with the psychological life of individuals, so must it be with the ages of his communal existence; these may be marked off from each other by the predominant play of one element, its force may overpower the others or take them into itself or make some compromise, but an exclusive play seems to be neither intended nor possible.”
Sri Aurobindo, The Human Cycle: The Psychology of Social Development, Chapter 18, The Infrarational Age of the Cycle, pp. 184-185