“When learned men begin to use their reason, then I generally discover that they haven't got any.”

If you cannot tolerate the planet that it is on? Grade the ground first. If a man believes and expects great things of himself, it makes no odds where you put him, or what you show him ... he will be surrounded by grandeur. He is in the condition of a healthy and hungry man, who says to himself, — How sweet this crust is!
Letter to Harrison Blake (20 May 1860); published in Familiar Letters (1865)

Proposals Relating to the Education of Youth in Pensilvania (1749), p. 22 http://dewey.library.upenn.edu/sceti/printedbooksNew/index.cfm?textID=franklin_youth&PagePosition=1; the statement relates to the teaching of History as a subject, and the last quoted paragraph concludes with the footnote "†": Public Disputes warm the Imagination, whet the Industry, and strengthen the natural Abilities.
Context: History will also afford frequent Opportunities of showing the Necessity of a Publick Religion, from its Usefulness to the Publick; the Advantage of a Religious Character among private Persons; the Mischiefs of Superstition, &c. and the Excellency of the above all others antient or modern.
History will also give Occasion to expatiate on the advantage of Civil Orders and Constitutions, how men and their properties are protected by joining in Societies and establishing Government; their Industry encouraged and rewarded, Arts invented, and Life made more comfortable: the Advantages of Liberty, Mischiefs of Licentiousness, Benefits arising from good Laws and a due Execution of Justice &c. Thus may the first Principles of sound Politics be fixed in the minds of youth.
On Historical occasions, Questions of Right and Wrong, Justice and Injustice, will naturally arise, and may be put to Youth, which they may debate in Conversation and in Writing. When they ardently desire of Victory, for the Sake of the Praise attending it, they will begin to feel the want, and be sensible of the use of the Use of Logic, or the Art of Reasoning to discover Truth, and of Arguing to defend it, and convince adversaries.

Variant: People are generally better persuaded by the reasons which they have themselves discovered than by those which have come into the mind of others.
Source: Pensees

Source: Busman's Honeymoon

4 Burr. Part IV., 2394.
Dissenting in Millar v Taylor (1769)

Induction and Analogy in Mathematics (1954)

Source: Letter to Harrison Blake (20 May 1860); published in Familiar Letters (1865)
Context: What is the use of a house if you haven't got a tolerable planet to put it on? — If you cannot tolerate the planet that it is on?
Context: Men and boys are learning all kinds of trades but how to make men of themselves. They learn to make houses; but they are not so well housed, they are not so contented in their houses, as the woodchucks in their holes. What is the use of a house if you haven't got a tolerable planet to put it on? — If you cannot tolerate the planet that it is on? Grade the ground first. If a man believes and expects great things of himself, it makes no odds where you put him, or what you show him … he will be surrounded by grandeur. He is in the condition of a healthy and hungry man, who says to himself, — How sweet this crust is!

Source: Gone with the Wind

Correspondance avec le pasteur Pfister, 1909-1939, Gallimard, 1991, p.103; as quoted in Altruism: The Power of Compassion to Change Yourself and the World by Matthieu Ricard
Attributed from posthumous publications