Traces of the past exist, and not traces of the future, only because entropy was low in the past. There can be no other reason, since the only source of the difference between past and future is the low entropy of the past. In order to leave a trace, it is necessary for something to become arrested, to stop moving, and this can happen only in an irreversible process—that is to say, by degrading energy into heat. In this way, computers heat up, the brain heats up, the meteors that fall into the moon heat it; even the goose quill of a medieval scribe in a Benedictine abbey heats a little the
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