This is found in the Curculio of Plautus. The adage warns us to make an early retreat from danger; whoever wishes to avoid a calamity should avoid what gives rise to it. Thus whoever wishes not to be corrupted should avoid bad company; if one does not wish to lie with a girl one should not kiss her. The poet adds at once, 'Smoke can burn nothing, but fire can.' Smoke is the sign of fire about to break out.