PROTEUS: ’Tis true. O heaven, were man
But constant, he were perfect; that one error
Fills him with faults, makes him run through all th’ sins;
Inconstancy falls off ere it begins.

William Shakespeare (1564-1616) English dramatist and poet

Two Gentlemen of Verona, Act 5, sc. 4, l. 118ff (5.4.118-121) (c. 1590)