"...we fall into the ridiculous contradiction of a mind that asserts total unity and proves by its very assertion its own difference and the diversity it claimed to resolve." As written in the irst chapter of The Myth Of Sisyphus.
More complete quote:
If man realized that the universe like him can love and suffer, he would be reconciled. If thought discovered in the shimmering mirrors of phenomena eternal relations capable of summing them up and summing themselves up in a single principle, then would be seen an intellectual joy of which the myth of the blessed would be but a ridiculous imitation. That nostalgia for unity, that appetite for the absolute illustrates the essential impulse of the human drama. But the fact of that nostalgia’s existence does not imply that it is to be immediately satisfied. For if, bridging the gulf that separates desire from conquest, we assert with Parmenides the reality of the One (whatever it may be), we fall into the ridiculous contradiction of a mind that asserts total unity and proves by its very assertion its own difference and the diversity it claimed to resolve. This other vicious circle is enough to stifle our hopes . . . So long as the mind keeps silent in the motionless world of its hopes, everything is reflected and arranged in the unity of its nostalgia. But with its first move this world cracks and tumbles: an infinite number of shimmering fragments is offered to the understanding.
He's saying that when a mind takes action - such as the action of asserting total unity, or any other action - it "proves by its very assertion its own difference," because other minds did not take the same action.
Camus was a disciple of Nietzsche and hence like him refuted any other dimension of the world other than the material. This is why Nietzsche said in Thus Spake Zarathrustha "Remain truthful to the earth, my brethren, by the power of your virtue". Camus is so faithful to this vision that he calls the metaphysics of anything above the earth "a nostalgia for unity".
For if ... we assert with Parmenides the reality of the One ... we fall into the ridiculous contradiction of a mind that asserts total unity and proves by it's very assertion it's own difference and the diversity it claimed to resolve.
Looking past the word-smithery here, he is saying something banal and is immediate once one has accepted the Parmenidian One: How is the world is full of multiplicity and yet is one? It is a also question that motivates a great deal of metaphysics as well as physics, For example, Leucippus and Democritus answered Parmenides by positing the notion of atoms.
Camus himself wasn't past falling "into the ridiculous contradiction of mind": He called Simone Weil, a Christian mystic, "the only great spirit of our times."