“To supply a thought is mental massage; but to evolve a thought of your own is an achievement. Thinking is a brain exercise — and no faculty grows save as it is exercised.”

[describing the historical causes of the modern tendency to make intellect the servant of alien interests]
The Integrity of the Intellect (July 1920)

Message to woman suffrage supporters (c. 1875)
1870s

Rich Dad Poor Dad: What the Rich Teach Their Kids About Money-That the Poor and the Middle Class Do Not!

Source: A Man of Law's Tale (1952), At the Scottish bar, p. 33

Source: Altered Carbon (2002), Chapter 41 (p. 493)

Independence Day speech (1828)
Context: Liberty means, not the mere voting at elections, but the free and fearless exercise of the mental faculties, and that self-possession which springs out of well-reasoned opinions and consistent practice. It is for them to honour principles rather than men — to commemorate events rather than days; when they rejoice, to know for what they rejoice, and to rejoice only for what has brought, and what brings, peace and happiness to men. The event we commemorate this day has procured much of both, and shall procure, in the onward course of human improvement, more than we can now conceive of. For this — for the good obtained, and yet in store for our race — let us rejoice! But let us rejoice as men, not as children — as human beings, rather than as Americans — as reasoning beings, not as ignorants. So shall we rejoice to good purpose and in good feeling; so shall we improve the victory once on this day achieved, until all mankind hold with us the jubilee of independence.

1780s, Letter to Peter Carr (1787)

Source: On Liberty

Les objets extérieurs ont une action réelle sur le cerveau. Qui s’enferme entre quatre murs finit par perdre la faculté d’associer les idées et les mots. Que de prisonniers cellulaires devenus imbéciles, sinon fous, par le défaut d’exercice des facultés pensantes.
Source: Journey to the Center of the Earth (1864), Ch. XXVI: The worst peril of all