Plato's Demand for the Correct Cause of Anything

Plato says we need to satisfy very strict criteria in ident fnd the correct cause for anything. The presence of the true cause shouldn't be compatible with other possible outcomes. For instance, one man is taller than another by a head, because it's also possible to be smaller by a head. Ugliness must be caused either by another form, the form of ugly, or by a simple failure to resemble the form of beauty.