In this discussion wahm is not a name for all imagination. It denotes a particular estimating faculty and judgments arising from its habits. A feature often accompanying sensible things may wrongly be taken as necessary for everything.
For example, many ordinary objects have bodies, occupy space, and can be pointed at. It does not follow that “everything that exists must be pointable.” The observations concern particular sensible entities, whereas the conclusion concerns all existents.
Sound reasoning keeps the scope of evidence aligned with the scope of the conclusion; it does not transfer conditions of the sensible to the non-sensible without justification.