“Thought depends on the habitual exercise of the speculative faculties; action, on the determination of the will. The one assigns reasons for things, the other puts causes into act. … Such is the effeminacy of the speculative and philosophical temperament, compared with the promptness and vigour of the practical! … Reasoners in general are undecided, wavering, and sceptical, or yield at last to the weakest motive as most congenial to their feeble habit of soul.”

Un Art de Vivre (The Art of Living) (1939), The Art of Thinking

Ostriker, J.P. and Mitton, Simon (2013) Heart of Darkness p.9.
De Magnete (1600), quote about science

Die Scheu vor der Spekulation, das angebliche Forteilen vom bloß Theoretischen zum Praktischen, bewirkt im Handeln notwendig die gleiche Flachheit wie im Wissen. Das Studium einer streng theoretischen Philosophie macht uns am unmittelbarsten mit Ideen vertraut, und nur Ideen geben dem Handeln Nachdruck und sittliche Bedeutung.
Vorlesungen über die Methode des akademischen Studiums ( Seventh Lecture http://www.zeno.org/Philosophie/M/Schelling,+Friedrich+Wilhelm+Joseph/Vorlesungen+%C3%BCber+die+Methode+des+akademischen+Studiums/7.+%C3%9Cber+einige+%C3%A4u%C3%9Fere+Gegens%C3%A4tze+der+Philosophie,+vornehmlich+den+der+positiven+Wissenschaften), Friedrich Wilhelm Joseph von Schellings sämmtliche Werke, V, 1859, p. 277 http://babel.hathitrust.org/cgi/pt?q1=%22nur%20Ideen%20geben%22;id=uva.x002624295;view=1up;seq=301;start=1;sz=10;page=search;num=277.
On University Studies (1803)

Article on Philosophy, Vol. 25, p. 667, as quoted in Main Currents of Western Thought : Readings in Western European Intellectual History from the Middle Ages to the Present (1978) by Franklin Le Van Baumer
Variant translation: Reason is to the philosopher what grace is to the Christian. Grace moves the Christian to act, reason moves the philosopher. Other men walk in darkness; the philosopher, who has the same passions, acts only after reflection; he walks through the night, but it is preceded by a torch. The philosopher forms his principles on an infinity of particular observations. … He does not confuse truth with plausibility; he takes for truth what is true, for forgery what is false, for doubtful what is doubtful, and probable what is probable. … The philosophical spirit is thus a spirit of observation and accuracy.
L'Encyclopédie (1751-1766)
Context: Reason is to the philosopher what grace is to the Christian.
Grace causes the Christian to act, reason the philosopher. Other men are carried away by their passions, their actions not being preceded by reflection: these are the men who walk in darkness. On the other hand, the philosopher, even in his passions, acts only after reflection; he walks in the dark, but by a torch.
The philosopher forms his principles from an infinity of particular observations. Most people adopt principles without thinking of the observations that have produced them, they believe the maxims exist, so to speak, by themselves. But the philosopher takes maxims from their source; he examines their origin; he knows their proper value, and he makes use of them only in so far as they suit him.
Truth is not for the philosopher a mistress who corrupts his imagination and whom he believes to be found everywhere; he contents himself with being able to unravel it where he can perceive it. He does not confound it with probability; he takes for true what is true, for false what is false, for doubtful what is doubtful, and probable what is only probable. He does more, and here you have a great perfection of the philosopher: when he has no reason by which to judge, he knows how to live in suspension of judgment...
The philosophical spirit is, then, a spirit of observation and exactness, which relates everything to true principles...

Book I, 1369a.5
Rhetoric
Variant: All human actions have one or more of these seven causes: chance, nature, compulsions, habit, reason, passion and desire
Source: Selected Works

Source: Science and Statistics (1976), p. 791

Rationalism in Politics and Other Essays (1962)

Letter 56 (60), to Hugo Boxel (1674) http://oll.libertyfund.org/?option=com_staticxt&staticfile=show.php%3Ftitle=1711&chapter=144218&layout=html&Itemid=27
Source: The Letters
Context: When you say that if I deny, that the operations of seeing, hearing, attending, wishing, &c., can be ascribed to God, or that they exist in him in any eminent fashion, you do not know what sort of God mine is; I suspect that you believe there is no greater perfection than such as can be explained by the aforesaid attributes. I am not astonished; for I believe that, if a triangle could speak, it would say, in like manner, that God is eminently triangular, while a circle would say that the divine nature is eminently circular. Thus each would ascribe to God its own attributes, would assume itself to be like God, and look on everything else as ill-shaped.
The briefness of a letter and want of time do not allow me to enter into my opinion on the divine nature, or the questions you have propounded. Besides, suggesting difficulties is not the same as producing reasons. That we do many things in the world from conjecture is true, but that our redactions are based on conjecture is false. In practical life we are compelled to follow what is most probable; in speculative thought we are compelled to follow truth. A man would perish of hunger and thirst, if he refused to eat or drink, till he had obtained positive proof that food and drink would be good for him. But in philosophic reflection this is not so. On the contrary, we must take care not to admit as true anything, which is only probable. For when one falsity has been let in, infinite others follow.
Again, we cannot infer that because sciences of things divine and human are full of controversies and quarrels, therefore their whole subject-matter is uncertain; for there have been many persons so enamoured of contradiction, as to turn into ridicule geometrical axioms.

Book I, 1.121-[5]
History of the Peloponnesian War, Book I