Affirming the Consequent

See formal fallacies.

This fallacy has the following form:

Just because Q is the case, it does not imply that P is the case. This is a formal non sequitur that corresponds to the informal causal fallacy.

Why doesn’t this undermine Trancendental Arguments?

This does not undermine trancendental arguments as there is a difference between “necessary preconditions” and “pretended two-way implication” (see modal logic).