“And this [experimental] science verifies all natural and man-made things in particular, and in their appropriate discipline, by the experimental perfection, not by arguments of the still purely speculative sciences, nor through the weak, and imperfect experiences of practical knowledge. And therefore, this is the matron of all preceding sciences, and the final end of all speculation.”

Source: Opus Tertium, c. 1267, Ch. 14 as quoted in J. H. Bridges, The 'Opus Majus' of Roger Bacon (1900) Vol.1 http://books.google.com/books?id=6F0XAQAAMAAJ Preface pp.x-xi
Context: All these foregoing sciences are, properly speaking, speculative. There is indeed in every science a practical side, as Avicenna teaches in the first book of his Art of Medicine. Nevertheless, of Moral Philosophy alone can it be said that it is in the special and autonomatic sense practical, dealing as it does with human conduct with reference to virtue and vice, beatitude and misery. All other sciences are called speculative: they are not concerned with the deeds of the present or future life affecting man's salvation or damnation. All procedures of art and of nature are directed to these moral actions, and exist on account of them. They are of no account except in that they help forward right action. Thus practical and operative sciences, as experimental alchemy and the rest, are regarded as speculative in reference to the operations with which moral or political science is concerned. This science is the mistress of every department of philosophy. It employs and controls them for the advantage of states and kingdoms. It directs the choice of men who are to study in sciences and arts for the common good. It orders all members of the state or kingdom so that none shall remain without his proper work.

Introduction à l'Étude de la Médecine Expérimentale (1865)

Source: The God of Jane: A Psychic Manifesto (1981), p. 58
Context: When we believe that science or religion "has the truth," we stop our speculations. While still referring to the theory of evolution, science accepts it as a fact, about existence, and therefore any speculation that threatens that theory becomes almost heretical. So often it seems that there is no other choice in the matter of man's origin than a meaningless universe and an earth populated by creatures who fight for survival, or a universe created by Christianity's objectified God. And to me, at least, the Eastern religions present no acceptable answers, either.

Great Books: The Foundation of a Liberal Education (1954)

"On the Harmony of Theory and Practice in Mechanics" (Jan. 3, 1856)
Context: The evil influence of the supposed inconsistency of theory and practice upon speculative science, although much less conspicuous than it was in the ancient and middle ages, is still occasionally to be traced. This it is which opposes the mutual communication of ideas between men of science and men of practice, and which leads scientific men sometimes to employ, on problems that can only be regarded as ingenious mathematical exercises, much time and mental exertion that would be better bestowed on questions having some connection with the arts, and sometimes to state the results of really important investigations on practical subjects in a form too abstruse for ordinary use; so that the benefit which might be derived from their application is for years lost to the public; and valuable practical principles which might have been anticipated by reasoning, are left to be discovered by slow and costly experience.<!--pp. 175

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.

Nobel prize lecture

Source: Autobiography (1873)
Source: https://archive.org/details/autobiography01mill/page/160/mode/1up pp. 160-161

On the Social State of Marxism (1978)