“Mathematicians have tried in vain to this day to discover some order in the sequence of prime numbers, and we have reason to believe that it is a mystery into which the human mind will never penetrate.”

Speech (12 January 1968) http://www.cuba.cu/gobierno/discursos/1968/esp/f120168e.html

Original: La mente umana è un mondo a parte. Complesso, misterioso e mai conosciuto completamente. Per questo motivo, intrigherà sempre l'anima, il corpo ed un'altra mente.
Source: prevale.net

Pt. I, sec. 3, "The Principle of Economy Applied to Sentences"
The Philosophy of Style (1852)
Context: We have a priori reasons for believing that in every sentence there is some one order of words more effective than any other; and that this order is the one which presents the elements of the proposition in the succession in which they may be most readily put together.

The Shorter Leibniz Texts (2006) http://books.google.com/books?id=oFoCY3xJ8nkC&dq edited by Lloyd H. Strickland, p. 111

Rules for the Direction of the Mind: X.379
As quoted in [Clarke, Desmond M., 2006, Descartes : a Biography, Cambridge Press, 67, ISBN 978-0-521-82301-2]

Statement of 2006, partly cited in Stop Making Sense: Music from the Perspective of the Real (2015) by Scott Wilson, p. 117
2000s
Context: You don't have to be a mathematician to have a feel for numbers. A movie, by the way, was made — sort of a small-scale offbeat movie — called Pi recently. I think it starts off with a big string of digits running across the screen, and then there are people who get concerned with various things, and in the end this Bible code idea comes up. And that ties in with numbers, so the relation to numbers is not necessarily scientific, and even when I was mentally disturbed, I had a lot of interest in numbers.

Source: The Thing (1929), Ch. IV : The Drift From Domesticity
Context: In the matter of reforming things, as distinct from deforming them, there is one plain and simple principle; a principle which will probably be called a paradox. There exists in such a case a certain institution or law; let us say, for the sake of simplicity, a fence or gate erected across a road. The more modern type of reformer goes gaily up to it and says, "I don't see the use of this; let us clear it away." To which the more intelligent type of reformer will do well to answer: "If you don't see the use of it, I certainly won't let you clear it away. Go away and think. Then, when you can come back and tell me that you do see the use of it, I may allow you to destroy it."
This paradox rests on the most elementary common sense. The gate or fence did not grow there. It was not set up by somnambulists who built it in their sleep. It is highly improbable that it was put there by escaped lunatics who were for some reason loose in the street. Some person had some reason for thinking it would be a good thing for somebody. And until we know what the reason was, we really cannot judge whether the reason was reasonable. It is extremely probable that we have overlooked some whole aspect of the question, if something set up by human beings like ourselves seems to be entirely meaningless and mysterious. There are reformers who get over this difficulty by assuming that all their fathers were fools; but if that be so, we can only say that folly appears to be a hereditary disease. But the truth is that nobody has any business to destroy a social institution until he has really seen it as an historical institution. If he knows how it arose, and what purposes it was supposed to serve, he may really be able to say that they were bad purposes, or that they have since become bad purposes, or that they are purposes which are no longer served. But if he simply stares at the thing as a senseless monstrosity that has somehow sprung up in his path, it is he and not the traditionalist who is suffering from an illusion.

2000

The Unspeakable McInch (p. 39; all ellipses in the original)
Short fiction, The Many Worlds of Magnus Ridolph (1966)