Full Idea
Modus ponens is intuitively valid, but in A,A→B|B if A is true and B is false that must be because A→B is false. So A→B is false when A is true and B is false.
Gist of Idea
Modus ponens requires that A→B is F when A is T and B is F
Source
Frank Jackson (Conditionals [2006], 'Equiv')
Book Reference
'Blackwell Guide to Philosophy of Language', ed/tr. Devitt,M/Hanley,R [Blackwell 2006], p.213
A Reaction
This is his first step in showing how the truth functional account of A→B acquires its truth table. If you are giving up the truth functional view of conditionals, presumably you are not also going to give up modus ponens?
Related Ideas
Idea 14354 When A and B have the same truth value, A→B is true, because A→A is a logical truth [Jackson]
Idea 14355 (A&B)→A is a logical truth, even if antecedent false and consequent true, so it is T if A is F and B is T [Jackson]