aṣlaḥ
“The best” for the creature. Some Muʿtazilīs held God is obligated to do what is al-aṣlaḥ; the Māturīdī reply denies any such prior claim — God may create or withhold beneficence without injustice, so no universal aṣlaḥ obligation constrains what wisdom selects.
Arabic aṣlaḥ, superlative of ṣāliḥ, “sound, good” — “the most fitting”.
Aṣlaḥ — "the best" for the creature — names a Muʿtazilī obligation thesis: that justice requires God to do what is most fitting for His servants. It was used to explain hard cases, for instance that a child dies young because of foreknown future unbelief while a foreknown apostate is left alive.
The Māturīdī reply, worked out in Nasafī's al-Tamhīd, is denial of the premise: creatures possess no prior claim-right to beneficence, so withholding it is not injustice, and no universal aṣlaḥ obligation constrains what divine wisdom selects. Notably, the child/apostate comparison is deployed in the text to refute the universal obligation — not to state a positive criterion.
That is the residual the corpus tracks: refuting the obligation does not supply a non-ad-hoc criterion for when wisdom creates a foreknown harmful token versus preventing a feasible one. The strongest prevention cases the texts offer are prophecy and saintly charism — extraordinary, not ordinary policy.
The mistaken everyday reading is "the Muslims thought God must be nice." The obligation thesis was one school's, and its rivals dismantled it from inside the same tradition.