Full Idea
(A→A) is a logical truth, so some conditionals with antecedent and consequent the same truth value are true. But if '→' is a truth function, that will be true for all cases. Hence whenever A and B are alike in truth value, (A→B) is true.
Gist of Idea
When A and B have the same truth value, A→B is true, because A→A is a logical truth
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
His second step in demonstrating the truth table for →, assuming it is truth functional.
Related Ideas
Idea 14353 Modus ponens requires that A→B is F when A is T and B is F [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]