Vann McGee’s scientific contributions

... 14 Then if we are certain there is no sharp cutoff, the orthological approach would commit us to certainty in ¬p k ∨ p k+1 for each k, in which case Or-to-If would commit us to certainty in p k → p k+1 for each k; hence we would have to deny modus ponens for → in order to block the derivation of p n . Though there may be reasons to deny modus ponens for conditionals whose consequents contain epistemic modals or conditionals [28], being forced to deny modus ponens for simple conditionalsalbeit with vague predicates-may seem more costly. Perhaps a proponent of the orthological approach to vagueness could escape this modus ponens problem by denying that we are certain that there are no sharp cutoffs, or by denying that Or-to-If preserves certainty when vague predicates are involved, inspired by Fine's rejection of the move from ¬(p k ∧ ¬p k+1 ) to p k → p k+1 . ...

Reference:

Vagueness and the Connectives