“When the dialectical method is applied to the study of economic problems, economic phenomena are not viewed separately from each other, by bits and pieces, but in their inner connection as an integrated totality, structured around, and by, a basic predominant mode of production. This totality is analysed in all its aspects and manifestations, as determined by certain given laws of motion, which relate also to its origins and its inevitable disappearance. These laws of motion of the given mode of production are discovered to be nothing but the unfolding of the inner contradictions of that structure, which define its very nature. The given economic structure is seen to be characterized at one and the same time by the unity of these contradictions and by their struggle, both of which determine the constant changes which it undergoes.”

Source: A Manual of the Steam Engine and Other Prime Movers (1859), p. 31
Context: Hypothesis Of Molecular Vortices. In thermodynamics as well as in other branches of molecular physics, the laws of phenomena have to a certain extent been anticipated, and their investigation facilitated, by the aid of hypotheses as to occult molecular structures and motions with which such phenomena are assumed to be connected. The hypothesis which has answered that purpose in the case of thermodynamics, is called that of "molecular vortices," or otherwise, the "centrifugal theory of elasticity. (On this subject, see the Edinburgh Philosophical Journal, 1849; Edinburgh Transactions, vol. xx.; and Philosophical Magazine, passim, especially for December, 1851, and November and December, 1855.)

Source: Productive thinking, 1945, p. 84

Preface to ' (1859).
Source: A Contribution to the Critique of Political Economy
Context: In the social production of their life, men enter into definite relations that are indispensable and independent of their will; these relations of production correspond to a definite stage of development of their material forces of production. The sum total of these relations of production constitutes the economic structure of society — the real foundation, on which rises a legal and political superstructure and to which correspond definite forms of social consciousness. The mode of production of material life determines the social, political and intellectual life process in general. It is not the consciousness of men that determines their being, but, on the contrary, their social being that determines their consciousness. [Es ist nicht das Bewußtsein der Menschen, das ihr Sein, sondern umgekehrt ihr gesellschaftliches Sein, das ihr Bewusstsein bestimmt. ] At a certain stage of their development, the material productive forces in society come in conflict with the existing relations of production, or — what is but a legal expression for the same thing — with the property relations within which they have been at work before. From forms of development of the productive forces these relations turn into fetters. Then begins an epoch of social revolution. With the change of the economic foundation the entire immense superstructure is more or less rapidly transformed. In considering such transformations a distinction should always be made between the material transformation of the economic conditions of production, which can be determined with the precision of natural science, and the legal, political, religious, aesthetic or philosophic — in short, ideological forms in which men become conscious of this conflict and fight it out. Just as our opinion of an individual is not based on what he thinks of himself, so we can not judge of such a period of transformation by its own consciousness; on the contrary, this consciousness must be explained rather from the contradictions of material life, from the existing conflict between the social productive forces and the relations of production. No social order ever disappears before all the productive forces for which there is room in it have been developed; and new, higher relations of production never appear before the material conditions of their existence have matured in the womb of the old society itself. Therefore, mankind always sets itself only such tasks as it can solve; since, looking at the matter more closely, we will always find that the task itself arises only when the material conditions necessary for its solution already exist or are at least in the process of formation. In broad outlines we can designate the Asiatic, the ancient, the feudal, and the modern bourgeois modes of production as so many progressive epochs in the economic formation of society. The bourgeois relations of production are the last antagonistic form of the social process of production — antagonistic not in the sense of individual antagonism, but of one arising from the social conditions of life of the individuals; at the same time the productive forces developing in the womb of bourgeois society create the material conditions for the solution of that antagonism. This social formation constitutes, therefore, the closing chapter of the prehistoric stage of human society.

Source: Researches into the Mathematical Principles of the Theory of Wealth, 1897, p. 137

Reflections on the Motive Power of Heat (1824)

Introduction to Capital. Introduction to volume 1 (1976)

Hess to Herzen, March 1850, Briefwechsel p. 253
Hess' Diary

Source: Virtual Mercury House. Planetary & Interplanetary Events, p. 130

Spectrum: From Right to Left in the World of Ideas (2005), Ch. 5. "Norming Facts, Jürgen Habermas" (2004)