All our past life is there, preserved even to the most infinitesimal details, and that we forget nothing, and that all that we have felt, perceived, thought, willed, from the first awakening of our consciousness, survives indestructibly. But the memories which are preserved in these obscure depths are there in the state of invisible phantoms. They aspire, perhaps, to the light, but they do not even try to rise to it; they know that it is impossible and that I, as a living and acting being, have something else to do than to occupy myself with them. But suppose that, at a given moment, I become disinterested in the present situation, in the present action, in other words, that I am asleep. Then these memories, perceiving that I have taken away the obstacle, have raised the trapdoor which has kept them beneath the floor of consciousness, arise from the depths; they rise, they move. They rush together to the door which has been left ajar. They all want to get through. But they cannot; there are too many of them. From the multitudes which are called, which will be chosen? It is not hard to say. Formerly, when I was awake, the memories which forced their way were those which could involve claims of relationship with the present situation, with what I saw and heard around me. Now it is more vague. So, then, among the phantom memories which aspire to fill themselves with materiality, the only ones that succeed are those which can assimilate themselves with the color-dust that we perceive, the external and internal sensations that we catch, etc., and which, besides, respond to the affective tone of our general sensibility.

What is the underlying theme of the passage?