Quote from Frankl:
What does Spinoza say in his Ethics? –“Affectus, qui passio est, desinit esse passio simulatque eius claram et distinctam formamus ideam.” Emotion, which is suffering, ceases to be suffering as soon as we form a clear and precise picture of it.
This is from the Ethics, Part V, Prop III (Latin). The Elwes translation is available on Gutenberg.org:
PROP. III. An emotion, which is a passion, ceases to be a passion, as soon as we form a clear and distinct idea thereof.
Proof.--An emotion, which is a passion, is a confused idea (by the general Def. of the Emotions). If, therefore, we form a clear and distinct idea of a given emotion, that idea will only be distinguished from the emotion, in so far as it is referred to the mind only, by reason (II. xxi., and note); therefore (III. iii.), the emotion will cease to be a passion. Q.E.D.
Corollary--An emotion therefore becomes more under our control, and the mind is less passive in respect to it, in proportion as it is more known to us.