on justifying modus ponens.

Any investigation into the “grounds” of modus ponens—viz. the justification of using the rule—will assume its validity. Therefore, such an investigation is at best hopelessly circular and at worst an exercise in defending the obvious via the dubious.

To what shall we liken the two sorts of men who attempt to justify modus ponens? Each is like a king who wishes to fortify his castle. The first king commands his subjects to remove each stone and put it back in its original place. The second king commands his subjects to rebuild the castle upon straw.