to an _idea_. In that case, the antecedent must be supplied; as,
_Wrong_: If any one wishes to contribute to the cause, let him send
_it_ in the enclosed envelope.
_Right_: If any one wishes to contribute to the cause, let him send
_his contribution_ in the enclosed envelope.
_Wrong_: I wouldn't wear mittens. Nobody does _that_ nowadays.
_Right_: I wouldn't wear mittens. Nobody wears _them_ nowadays.