Why is that which is spoken of expressed as in + ablative?

Question

Vīta est spīrandī mūnere fruī, mors prīvārī. Hoc autem spīrandī mūnus apud plērōsque in bonīs dicitur.

Livet er å nyte Guds* åndingsgave; døden å berøves [den]. Denne pustingas gave anses dermed i de flestes øyne for et gode.)

Life is to enjoy God’s gift of breath; death is to be robbed [of it]. This gift of breathing is by most people considered a good.

― Ambrosius, Dē bonō mortis 1.2.

Now why is it in bonīs instead of simply an accusative? (And why the plural? Other places in the text, he does not seem to have a need for the plural.) I have not been able to find any reasonable explanations for the usage of ablative in such a case in my grammar, nor have I been able to find any relevant examples in my dictionary.

* In classical times, this was to be understood as ‘the gifts from nature and the gods’ (Latinsk ordbok: «mūnus»).

Answer 1

In bonis means "among (the) good things": that is, the gift of breath is considered to be among / in the category of things that are good.

Bonum could have been used and would be grammatically correct, but gives a slightly different meaning, simply "considered good / a good thing" without any added sense that there is a definable class of "good things" which it is a member of.