More precisely still, the relevant belief must be that intending the means is itself necessary.  Suppose your dentist intends to remove your tooth, and believes that a necessary means to removing your tooth is causing you pain.  Is she being instrumentally incoherent in refusing to intend to cause you pain? Not necessarily: she might believe that she will remove your tooth even if she does not intend to cause you pain.  But she will, presumably, believe that she will remove your tooth only if she intends to use her instruments.  So, though she wouldn’t be irrational in not intending to cause you pain, she would be irrational in not intending to use her instruments (Broome 2003; Searle 2001; Kamm 2000, 2007).