“Communism differs from all previous movements in that it overturns the basis of all earlier relations of production and intercourse, and for the first time consciously treats all natural premises as the creatures of hitherto existing men, strips them of their natural character and subjugates them to the power of the united individuals. Its organisation is, therefore, essentially economic, the material production of the conditions of this unity; it turns existing conditions into conditions of unity. The reality, which communism is creating, is precisely the true basis for rendering it impossible that anything should exist independently of individuals, insofar as reality is only a product of the preceding intercourse of individuals themselves.”

Source: The German Ideology (1845-1846), Volume I; Part 1; "Feuerbach. Opposition of the Materialist and Idealist Outlook"; Section A, "Idealism and Materialism".

Section 1, paragraph 18, lines 6-9.
The Manifesto of the Communist Party (1848)

The Development Hypothesis (1852)
Context: The supporters of the Development Hypothesis... can show that any existing species—animal or vegetable—when placed under conditions different from its previous ones, immediately begins to undergo certain changes fitting it for the new conditions. They can show that in successive generations these changes continue; until, ultimately, the new conditions become the natural ones. They can show that in cultivated plants, in domesticated animals, and in the several races of men, such alterations have taken place. They can show that the degrees of difference so produced are often, as in dogs, greater than those on which distinctions of species are in other cases founded.

real, active men, as they are conditioned by a definite development of their productive forces and of the intercourse corresponding to these, up to its furthest forms. Consciousness can never be anything else than conscious existence, and the existence of men is their actual life-process. If in all ideology men and their circumstances appear upside down as in a camera obscura, this phenomenon arises just as much from their historical life-process as the inversion of objects on the retina does from their physical life-process.
Source: The German Ideology (1845-1846)

Vol. I, Part 4.
The German Ideology (1845/46)
Context: Communism differs from all previous movements in that it overturns the basis of all earlier relations of production and intercourse, and for the first time consciously treats all natural premises as the creatures of hitherto existing men, strips them of their natural character and subjugates them to the power of the united individuals. Its organisation is, therefore, essentially economic, the material production of the conditions of this unity; it turns existing conditions into conditions of unity. The reality, which communism is creating, is precisely the true basis for rendering it impossible that anything should exist independently of individuals, insofar as reality is only a product of the preceding intercourse of individuals themselves.

The German Ideology (1845/46)
Context: The fact is, therefore, that definite individuals who are productively active in a definite way enter into these definite social and political relations. Empirical observation must in each separate instance bring out empirically, and without any mystification and speculation, the connection of the social and political structure with production. The social structure and the state are continually evolving out of the life-process of definite individuals, but of individuals, not as they appear in their own or other people's imagination, but as they really are; i. e. as they are effective, produce materially, and are active under definite material limits, presuppositions and conditions independent of their will.
The production of ideas, of conceptions, of consciousness, is at first directly interwoven with the material activity and the material intercourse of men, the language of real life. Conceiving, thinking, the mental intercourse of men, appear at this stage as the direct efflux of their material behaviour. The same applies to mental production as expressed in the language of the politics, laws, morality, religion, metaphysics of a people. Men are the producers of their conception, ideas, etc. — real, active men, as they are conditioned by a definite development of their productive forces and of the intercourse corresponding to these, up to its furthest forms. Consciousness can never be anything else than conscious existence, and the existence of men is their actual life-process. If in all ideology men and their circumstances appear upside down as in a camera obscura, this phenomenon arises just as much from their historical life-process as the inversion of objects on the retina does from their physical life-process.

Source: The German Ideology (1845-1846), Vol. I, Part 1.

Source: Psychology: An elementary textbook, 1908, p. 44

Guerrilla Warfare (1960)