I have a follow-up question:
qua noceat tantum aspicit. (De Ira, 2.14.3)
You already told me the meaning. I have a question about the grammar. Since “qua” is an adv, “qua noceat” is not a clause after “aspicit”. So I don’t quite understand how we can put two verbs, which are not parallel verbs, next to each other. I thought the second verb usually is an inf.
In “….qua noceat tantum aspicit” (Seneca, De Ira, book II, chapter 14, section 3 ) the ablative “quā” (literally, “by what way “) is used as an interrogative adverb/conjunction just to introduce the indirect question clause “qua noceat” which depends on “aspicit”, so that “aspicit”, which means”looks”/”considers” (whose subject is “ira”), governs the indirect question clause “quā noceat” which takes the subjunctive “noceat”, like all the indirect question clauses that in Latin take the subjunctive.
To sum up, “quā noceat” is exactly a clause after “aspicit” and “Ira enim perturbat artem et qua noceat tantum aspicit” means:”Anger in fact disturbs/confounds athletic skills and looks/considers only by what way/ how it (i.e. anger) can injure/harm”.
So, as you know, the indirect question clauses are introduced by special interrogative words which are followed by the subjunctive (see AG 331 and 574), according to Consecutio Temporum (Sequence of tenses).
To conclude, “noceat” and “aspicit” are not parallel verbs, as “aspicit” is the verb of the main clause, while “noceat” is the present subjunctive of the subordinate indirect question clause introduced by the relative/interrogative adverb/conjunction “quā”(abl.fem. of "qui/quis") .
As for your sentence “I thought the second verb usually is an infinitive", this is true only when we have a verb which takes the infinitive as in e.g. “Id facere possum”, “Proficisci volo”, Te videre nolo”, etc.
Hope all is clear enough.