This is a quote from the book "The ​Virtue Of Selfishness" by Ayn Rand
...first step in the realm of evaluation.

The capacity to experience pleasure or pain is innate in a man’s body; it is part of his nature, part of the kind of entity he is. He has n
o choice about it, and he has no choice about the standard that determines what will make him experience the physical sensation of pleasure or of pain. What is that standard? His life.
r /> The pleasure-pain mechanism in the body of man — and in the bodies of all the living organisms that possess the faculty of consciousness — serves as an automatic guardian of the organis...
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