The Upanishads, Part 1 (SBE01)
Chapters
Khandogya-Upanishad: Fifth Prapathaka: Tenth Khanda

Another curious distinction is here made by Sankara in his commentary. There are some, he says, who assume the form of rice, corn, &c., not in their descent from a higher world, as described in the Upanishad, but as a definite punishment for certain evil deeds they have committed. These remain in that state till the results of their evil deeds are over, and assume then a new body, according to their work, like caterpillars. With them there is also a consciousness of these states, and the acts which caused them to assume this or that body, leave impressions behind, like dreams. This is not the case with those who in their descent from the moon, pass, as we saw, through an existence as rice, corn, &c. They have no consciousness of such existences, at least not in their descent. In their ascent to the moon, they have consciousness, as a man who climbs up a tree knows what he is about. But in their descent, that consciousness is gone, as it is when a man falls down from a tree. Otherwise a man, who by his good works had deserved rewards in the moon, would, while corn is being ground, suffer tortures, as if he were in hell, and the very object of good works, as taught by the Veda, would be defeated. As we see that a man struck by a hammer can be carried away unconscious, so it is in the descent of souls, till they are born again as men, and gain a new start for the attainment of the Highest Brahman.

^83:1 Let him despise it. Comm.

^83:2 Evidently an old Trishtubh verse, but irregular in the third line. See Manu XI, 54.