 (Following Vann McGee (1989) I’ll call the principle that (i) and (ii) are equivalent the Import-Export Principle, or “Import-Export” for short.) Try any example: “If Mary comes then if John doesn’t have to leave early we will play Bridge”; “If Mary comes and John doesn’t have to leave early we will play Bridge”. “If they were outside and it rained, they got wet”; “If they were outside, then if it rained, they got wet”. For Hook, Import-Export holds. (Exercise: do a truth table, or construct a proof.) Gibbard (1981, pp. 234–5) has proved that for no conditional with truth conditions stronger than \(\supset\) does Import-Export hold. Assume Import-Export holds for some reading of “if”. The key to the proof is to consider the formula