“It is perilous to separate thinking rightly from acting rightly. He is already half false who speculates on truth and does not do it. Truth is given, not to be contemplated, but to be done. Life is an action — not a thought. And the penalty paid by him who speculates on truth, is that by degrees the very truth he holds becomes a falsehood.”

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.

Für die Romantiker und für die spekulative Philosophie bedeutete der Terminus kritisch: objektiv produktiv, schöpferisch aus Besonnenheit. Kritisch sein hieß die Erhebung des Denkens über alle Bindungen so weit treiben, daß gleichsam zauberisch aus der Einsicht in das Falsche der Bindungen die Erkenntnis der Wahrheit sich schwang.
The Concept of Criticism in German Romanticism (1919)

Henry J. Friendly, Mr. Justice Brandeis: The Quest for Reason, 108 U. Pa. L. Rev. 985, 999 (1960).

1840s, Heroes and Hero-Worship (1840), The Hero as Prophet

"Lettre du Provincial" (21 December 1899)
Basic Verities, Prose and Poetry (1943)

1900s, "The Study of Mathematics" (November 1907)
Context: Mathematics, rightly viewed, possesses not only truth, but supreme beauty – a beauty cold and austere, like that of sculpture, without appeal to any part of our weaker nature, without the gorgeous trappings of painting or music, yet sublimely pure, and capable of a stern perfection such as only the greatest art can show. The true spirit of delight, the exaltation, the sense of being more than Man, which is the touchstone of highest excellence, is to be found in mathematics as surely as in poetry. What is best in mathematics deserves not merely to be learnt as a task, but to be assimilated as a part of daily thought, and brought again and again before the mind with ever-renewed encouragement.

Le Discours de la Méthode (1637)