phrasal
1. : to be inescapably constrained to (as out of a sense of fitness or rightness) : be left with no alternative than to
an obsequiousness one cannot but feel aversion to
2. : to be bound to : be sure to : must
his personality cannot but come through in his letters
3. : to be unable to do otherwise than to
the outsider cannot but be struck by the frequent reluctance of the learned world to recognize important discoveries — Edmund Wilson
— compare but II 2b