Asphorisms collected; The Oxford Book of Aphorisms, compiled by John Gross. Oxford and New York: Oxford University Press. 383 pp. $15.95.

John Gross's ''Oxford Book of Aphorisms'' differs from many such collections by including longer passages, some of which lack the swift conclusiveness and, hence, the memorability of the classic aphorism. Still there are plenty of the kind we expect, if one can really be said to expect this tour de vengeance from G.K. Chesterton: ''Literature is a luxury; fiction is a necessity.''

One grants, albeit begrudgingly, the former proposition, only to be trapped into concluding the latter by force of style. The style of Chesterton's verbal whiplash is classic. Antithesis, paradox, balance; mirrored subject noun, linking verb, predicate noun, hinged (unhinged?) by the impassive but smiling semicolon - a surprising, an elegant, sentence.

On another page we find the following. ''That girl in the omnibus had one of those faces of marvelous beauty which are seen casually in the streets but never among one's friends. Where do these women come from? Who marries them? Who knows them?'' Thomas Hardy is reported to have said it, but is it an aphorism?

Again, the shock of recognition, this time the cry of the embarrassed heart. We see the point, confirming it, implicating ourselves. An ironic aphorism? A closer look reveals elements of the classical style in Hardy's speech. Antithesis is the key: ''casually . . . but not.'' We note next the sentence patterns, the longs and shorts. The long sentence is a thicket of short words spaced by some longer Latinate ones. The specificity of ''that girl,'' furthermore, becomes, a little later, generalized: ''these wo-men.'' Still, at first we seem to be eavesdropping (the effect of shared or unconscious intimacy is not unlike that of Hardy's lyrics). Then the series of short sentences, questions all. The final effect of this pattern of sentences, at least on me, is almost one of self-parody, for the speaker seems to be laughing at his own sense of helplessness.

But is it an aphorism? Editor Gross's eye appears to have been on the truth, the justice, of the observation. The justice is in the style. The style renders - preserves, presents, represents - the feeling appropriate to the observation and the irony. The whole thing is a bit embarrassing, but it won't go away. To marry such a woman may not be to know her; at any rate, to know her is not for the likes of you and me. Pure Hardy. Aphoristic or not, style is the man.