As explained by Rudolf Steiner
From the Lecture: “Behind the Scenes of External Events, part 2”
“In their souls, human beings more and more come to resemble the thought, to resemble that which they regard as knowledge. This will seem a strange truth to the modern mind, but it is so, nevertheless. (…)
For example; to regard Darwinism as the one and only valid conception of the world, believing the only possible truth to be that man descends from the animals–that I descend entirely from forces which also produce the animals… such thoughts, in our age, tend to make the soul resemble its own conceptions of itself.
When the body is discarded, the soul is then confronted with the sorry fate of having to perceive its resemblance with its own thought! A man who lives in the physical body believing that animal forces alone were at work in his evolution, fashions for himself a kind of consciousness in which he will perceive his own likeness to animal nature.
It is ordained that in times to come, what the human considers himself to BE, that he will BECOME.”
“What man thinks himself to BE, that he is obliged to BECOME… This is a truth that was destined, after the great changes in the nineteenth century, to find its way to men. The human being must be voluntarily anything that he can be really; he must be able to think about his own being if he is to be truly himself in his life of soul.”
“Spirits of darkness, who oppose Man’s destiny, inspired human beings to announce the following: ‘Man is what he eats’. And although this is not, in theory, widely acknowledged, the practical conduct of life amounts very nearly to being an acknowledgement of the principle that man is what he eats–that and nothing else.”
“If it is true that in yonder world man becomes what he has pictured himself to be, then something else is also true. For example: A man dies, leaving relatives behind him. Although thought may not be entirely lacking in these relatives, they may be materialistically minded, and then, quite inevitably, they will think either that the dead man is decaying in the grave or that what exists of him is preserved in the urn.
THIS THOUGHT IS A REAL POWER. It is an untruth. When those left behind think that the dead man is no longer there; this thought is real and actual in the souls of those who form it. And the dead man is aware of this thought-reality, is aware of its significance for him.
It is therefore a matter of fundamental importance whether those left behind cherish in their souls the thought of The Dead living on in the spiritual world, or whether they instead succumb to the woeful idea that the dead man… well, he is dead; he lies there decaying in the grave.”
– Zurich, 13 November, 1917