„The belief in an external world independent of the perceiving subject is the basis of all natural science.“

— Albert Einstein

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„Logic is the science not of external forms of thought, but of the laws of development "of all material, natural and spiritual things", i. e., of the development of the entire concrete content of the world and of its cognition, i. e., the sum-total, the conclusion of the History of knowledge of the world.“
— Vladimir Lenin