“[…] the virtues of a medicine depend less upon its intrinsic properties and powers than on the sagacity of the physician who administers it; just as the efficiency of firearms depends less upon the explosives and the missile they contain than on the judgment and accuracy of aim of the man who discharges them.”

Pt. I, sec. 1, "The Principle of Economy"
The Philosophy of Style (1852)
Context: There can be little question that good composition is far less dependent upon acquaintance with its laws, than upon practice and natural aptitude. A clear head, a quick imagination, and a sensitive ear, will go far towards making all rhetorical precepts needless.

Source: Political Aphorisms, Moral and Philosophical Thoughts (1848), p. 146

1860s, A Liberal Education and Where to Find It (1868)
Context: The life, the fortune, and the happiness of every one of us, and, more or less, of those who are connected with us, do depend upon our knowing something of the rules of a game infinitely more difficult and complicated than chess. It is a game which has been played for untold ages, every man and woman of us being one of the two players in a game of his or her own. The chessboard is the world, the pieces are the phenomena of the universe, the rules of the game are what we call the laws of Nature. The player on the other side is hidden from us. We know that his play is always fair, just, and patient. But also we know, to our cost, that he never overlooks a mistake, or makes the smallest allowance for ignorance. To the man who plays well, the highest stakes are paid, with that sort of overflowing generosity with which the strong shows delight in strength. And one who plays ill is checkmated — without haste, but without remorse.

The Heavenly City of the Eighteenth-century Philosophers (1932)

Source: 1940s, Frontiers in group dynamics II, 1947, p. 153.

Source: The Sands of Time

Address at Burlington College, reported in Horace Mann, The Common School Journal (1847), p. 191.

Le bonheur et le malheur des hommes ne dépend pas moins de leur humeur que de la fortune.
Maxim 61.
Reflections; or Sentences and Moral Maxims (1665–1678)

Source: 1940s–present, Minority Report : H.L. Mencken's Notebooks (1956), p. 217-218