The following statements makes sense
It is impossible to doubt that you exist.
It is impossible to doubt that your mind exists.
However, if I were to add parentheses to the first statement which contain the information in the second, it becomes unclear as to which form of
exist I should use.
It is impossible to doubt that you (or your mind) exist/exists.
Similarly and more confusingly:
You (or he) jump/jumps.