Letters 18.15

But indeed this emotion blazes out against all sorts of persons; it springs from love as much as from hate, and shows itself not less in serious matters than in jest and sport. And it makes no difference how important the provocation may be, but into what kind of soul it penetrates. Similarly with fire; it does not matter how great is the flame, but what it falls upon. For solid timbers have repelled a very great fire; conversely, dry and easily inflammable stuff nourishes the slightest spark into a conflagration. So it is with anger, my dear Lucilius; the outcome of a mighty anger is madness, and hence anger should be avoided, not merely that we may escape excess, but that we may have a healthy mind. Farewell.

Seneca, Letters to Lucilius 18.15, translated by Richard Mott Gummere