المشكل: ما ازداد خفاء على الخفي.
Mushkal is wording whose obscurity is greater than that of khafi.
With khafi, the ordinary meaning of the expression was clear; the problem arose in applying it to an unusual case. With mushkal, determining the intended usage itself requires a closer examination of meanings.
To resolve it, one first investigates the plausible senses and customary usages, then reflects on their relevance to the context. The source accordingly adds ta'ammul, careful interpretive reflection, to the requirement of inquiry.