“Knowledge begins with practice, and theoretical knowledge, which is acquired through practice, must then return to practice. The active function of knowledge manifests itself not only in the active leap from perceptual to rational knowledge, but - and this is more important - it must manifest itself in the leap from rational knowledge to revolutionary practice.”

"On the Harmony of Theory and Practice in Mechanics" (Jan. 3, 1856)

p, 125
"On the Harmony of Theory and Practice in Mechanics" (Jan. 3, 1856)

"On the Harmony of Theory and Practice in Mechanics" (Jan. 3, 1856)
Context: Another evil, and one of the worst which arises from the separation of theoretical and practical knowledge, is the fact that a large number of persons, possessed of an inventive turn of mind and of considerable skill in the manual operations of practical mechanics, are destitute of that knowledge of scientific principles which is requisite to prevent their being misled by their own ingenuity. Such men too often spend their money, waste their lives, and it may be lose their reason in the vain pursuits of visionary inventions, of which a moderate amount of theoretical knowledge would be sufficient to demonstrate the fallacy; and for want of such knowledge, many a man who might have been a useful and happy member of society, becomes a being than whom it would be hard to find anything more miserable.
The number of those unhappy persons — to judge from the patent-lists, and from some of the mechanical journals — must be much greater than is generally believed.<!--p. 176

Source: Sufi Thought and Action

Source: 1990s, Ideology (1991), p. 136

“German Volksgenossen!” Hitler’s opening speech at the new Winterhilfswerk, Deutschlandhalle, Berlin (October 5, 1937). Also quoted in The Third Reich: A New History by Michael Burleigh https://books.google.com/books?id=l5gcZpnL5QUC&pg=PA224
1930s

Vol. I, Part 1, [The Materialist Conception of History].
The German Ideology (1845/46)
Context: Where speculation ends — in real life — there real, positive science begins: the representation of the practical activity, of the practical process of development of men. Empty talk about consciousness ceases, and real knowledge has to take place. When reality is depicted, philosophy as an independent branch of activity loses its medium of existence. At the best its place can only be taken by a summing-up of the most general results, abstractions which arise from the observation of the historical development of men. Viewed apart from real history, these abstractions have in themselves no value whatsoever. They can only serve to facilitate the arrangement of historical material, to indicate the sequence of its separate strata. But they by no means afford a recipe or schema, as does philosophy, for neatly trimming the epochs of history. On the contrary, our difficulties begin only when we set about the observation and the arrangement — the real depiction — of our historical material, whether of a past epoch or of the present.

the necessary and sufficient conditions for rational knowledge
Source: Great Islamic Encyclopedia website, 2016 https://www.cgie.org.ir/fa/news/154958

David Sayre, while in a panel discussion with Hopper, as quoted in Management and the Computer of the Future (1962) by Sloan School of Management, p. 277
Misattributed