Zeno's definition, then, is this: "A perturbation" (which he calls a [Greek: pathos]) "is a commotion of the mind repugnant to reason, and against nature." Some of them define it even more briefly, saying that a perturbation is a somewhat too vehement appetite; but by too vehement they mean an appetite that recedes further from the constancy of nature.

But they would have the divisions of perturbations to arise from two imagined goods, and from two imagined evils; and thus they become four: from the good proceed lust and joy--joy having reference to some present good, and lust to some future one.

They suppose fear and grief to proceed from evils: fear from something future, grief from something present; for whatever things are dreaded as approaching always occasion grief when present.

But joy and lust depend on the opinion of good; as lust, being inflamed and provoked, is carried on eagerly towards what has the appearance of good; and joy is transported and exults on obtaining what was desired: for we naturally pursue those things that have the appearance of good, and avoid the contrary.
Wherefore, as soon as anything that has the appearance of good presents itself, nature incites us to endeavor to obtain it.

Now, where this strong desire is consistent and founded on prudence, it is by the Stoics called [Greek: boulesis], and the name which we give it is volition; and this they allow to none but their wise man, and define it thus: Volition is a reasonable desire; but whatever is incited too violently in opposition to reason, that is a lust, or an unbridled desire, which is discoverable in all fools.