Ayn Rand, Atlas Shrugged:

A is A. A thing is itself. You have never grasped the meaning of his statement. I am here to complete it….Whatever you choose to consider, be it an object, an attribute or an action, the law of identity remains the same. A leaf cannot be a stone at the same time, it cannot be all red and all green at the same time, it cannot freeze and burn at the same time. A is A. Or, if you wish it stated in simpler language: You cannot have your cake and eat it, too….All the disasters that have wrecked your world, came from your leaders’ attempt to evade the fact that A is A. All the secret evil you dread to face within you and all the pain you have ever endured, came from your own attempt to evade the fact that A is A. The purpose of those who taught you to evade it, was to make you forget that Man is Man.

Ludwig Wittgenstein, Philosophical Investigations:

“A thing is identical with itself.” — There is no finer example of a useless proposition.