Front Page Titles (by Subject) GRAVE—GRAVITY. - The Works of Voltaire, Vol. V (Philosophical Dictionary Part 3)
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GRAVE—GRAVITY. - 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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Grave, in its moral meaning, always corresponds with its physical one; it expresses something of weight; thus, we say—a person, an author, or a maxim of weight, for a grave person, author, or maxim. The grave is to the serious what the lively is to the agreeable. It is one degree more of the same thing, and that degree a considerable one. A man may be serious by temperament, and even from want of ideas. He is grave, either from a sense of decorum, or from having ideas of depth and importance, which induce gravity. There is a difference between being grave and being a grave man. It is a fault to be unseasonably grave. He who is grave in society is seldom much sought for; but a grave man is one who acquires influence and authority more by his real wisdom than his external carriage.
A decorous air should be always preserved, but a grave air is becoming only in the function of some high and important office, as, for example, in council. When gravity consists, as is frequently the case, only in the exterior carriage, frivolous remarks are delivered with a pompous solemnity, exciting at once ridicule and aversion. We do not easily pardon those who wish to impose upon us by this air of consequence and self-sufficiency.
The duke de La Rochefoucauld said “Gravity is a mysteriousness of body assumed in order to conceal defects of mind.” Without investigating whether the phrase “mysteriousness of body” is natural and judicious, it is sufficient to observe that the remark is applicable to all who affect gravity, but not to those who merely exhibit a gravity suitable to the office they hold, the place where they are, or the business in which they are engaged.
A grave author is one whose opinions relate to matters obviously disputable. We never apply the term to one who has written on subjects which admit no doubt or controversy. It would be ridiculous to call Euclid and Archimedes grave authors.
Gravity is applicable to style. Livy and de Thou have written with gravity. The same observations cannot with propriety be applied to Tacitus, whose object was brevity, and who has displayed malignity; still less can it be applied to Cardinal de Retz, who sometimes infuses into his writings a misplaced gayety, and sometimes even forgets decency.
The grave style declines all sallies of wit or pleasantry; if it sometimes reaches the sublime, if on any particular occasion it is pathetic, it speedily returns to the didactic wisdom and noble simplicity which habitually characterizes it; it possesses strength without daring. Its greatest difficulty is to avoid monotony.
A grave affair (affaire), a grave case (cas), is used concerning a criminal rather than a civil process. A grave disease implies danger.