Front Page Titles (by Subject) FIRMNESS. - The Works of Voltaire, Vol. V (Philosophical Dictionary Part 3)
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FIRMNESS. - Voltaire, The Works of Voltaire, Vol. V (Philosophical Dictionary Part 3) 
The Works of Voltaire. A Contemporary Version. A Critique and Biography by John Morley, notes by Tobias Smollett, trans. William F. Fleming (New York: E.R. DuMont, 1901). In 21 vols. Vol. V.
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Firmness comes from firm, and has a different signification from solidity and hardness; a squeezed cloth, a beaten negro, have firmness without being hard or solid.
It must always be remembered that modifications of the soul can only be expressed by physical images; we say firmness of soul, and of mind, which does not signify that they are harder or more solid than usual.
Firmness is the exercise of mental courage; it means a decided resolution; while obstinacy, on the contrary, signifies blindness. Those who praise the firmness of Tacitus are not so much in the wrong as P. Bouhours pretends; it is an accidental ill-chosen term, which expresses energy and strength of thought and of style. It may be said that La Bruyère has a firm style, and that many other writers have only a hard one.