Friedrich Engels, answered from the texts and cited to the page.
Scientific socialism is socialism grounded in the actual study of the historical-economic process by which capitalism produces its own conditions of supersession. Utopian socialism — Owen, Saint-Simon, Fourier — was indispensable in its time as moral critique and visionary anticipation, but it had no theory of how socialism could come about.12
The transition requires a class agent (the proletariat), a material base (the development of large-scale industry), and a contradiction internal to capitalism (the socialization of production confronting the private appropriation of its product). Marx and I supplied that theory. "Scientific" does not mean predictive in the manner of celestial mechanics; it means historically grounded, evidentiary, and capable of distinguishing what the developmental process makes possible from what mere wish makes desirable.34
The mode of production peculiar to the bourgeoisie, known, since Marx, as the capitalist mode of production, was incompatible with the feudal system, with the privileges it conferred upon individuals, entire social ranks and local corporations, as well as with the hereditary ties of subordination which constituted the framework of its social organisation.Socialism, Utopian and Scientific, pp. 47–49
Then came the three great Utopians: Saint Simon, to whom the middle-class movement, side by side with the proletarian, still had a certain significance; Fourier; and Owen, who in the country where capitalist production was most developed, and under the influence of the antagon- | isms begotten of this, worked out his proposals for the removal of class distinction systematically and in direct relation to French…Socialism, Utopian and Scientific, pp. 6–7
If Don Quixote tilts his lance at windmills, that is in keeping with his job and his role; but we cannot possibly allow Sancho Panza anything of the sort. Such readers will also be surprised to encounter the Kant-Laplace cosmogony, modern natural science and Darwin, classical German philosophy and Hegel in a sketch of the history of the development of socialism.Socialism, Utopian and Scientific, pp. 6–9
As soon as the demand is made upon each separate branch of science that it make clear its relation to things in general, and science as a whole, the individual science thereupon becomes superfluous. Of all philosophy up to the present time the only peculiar property which remains as its characteristic is the study of thought and the formal laws of thought--logic and the dialectic.Landmarks of Scientific Socialism - Anti-Duhring
The systematic exposition is in Anti-Dühring (1877–78); the popular distillation is Socialism: Utopian and Scientific (1880), which became the most widely read socialist text in any language during the late 19th century. I have not retreated under sustained Frankfurt-school critique that "scientific" smuggled in 19th-century positivism, nor under the revisionist (Bernstein) appropriation that read "scientific" as a warrant for evolutionary parliamentarism.