Cicero gives a nice rundown of the Stoic theory of emotions, which holds up well according to modern cognitive science. Emotions have cognitive components, so we can challenge them when they are not good for us.

If our concerns are in agreement with reason, they are healthy; but fear is not in agreement with reason, and it is therefore unhealthy.

Where this strong desire is consistent and founded on prudence, it is by the Stoics called volition. And this they define it thus: volition is a reasonable desire.

Zeno’s definition, then, is this: “A perturbation” (which he calls “pathos”) “is a commotion of the mind repugnant to reason, and against nature.”

The sage will achieve a state of apatheia, meaning lack of disturbance from unhealthy emotions like fear, anger, and hatred. But she will also experience healthy emotions, like love, joy, and a sense of justice.

Cicero very clearly and succinctly explains the difference among five Hellenistic takes on grief, including two Stoic ones, one by Cleanthes (the second head of the Stoa) and one by Chrysippus (the third head).

Grief arises from an opinion of some present evil, which includes this belief, that it is incumbent on us to grieve.

Most people appear to be unaware what contradictions these things are full of. They commend those who die calmly, but they blame those who can bear the loss of another with the same calmness.

Cicero gives an example of people suddenly setting grief aside because they are absorbed in an urgent task. He infers that, therefore, grief is a matter of opinion, not of nature.

You see, the evil is in opinion, not in nature.