Consider for a moment all that human beings think! Would it not be terrible if every human thought were objectively engraved in cosmic substance and remained there for ever and ever? That, however, is what would happen if man were not able, through repeated earthly lives, either to correct thoughts that are not worthy of permanence or entirely reject and substitute others for them. This is provided for in evolution by the repetition of earthly lives, for man is thus enabled to better what is impressed upon the etheric substance of the cosmos every time he passes through death and he can strive to the end that after his final incarnation on Earth only that will have been given over to the etheric substance of the cosmos which is worthy of permanence.
Source: Rudolf Steiner – GA 170 – Memory and Habit – Dornach, 27th August 1916