As Mr. Sadler points out, there can be no conflict between mathematicians and navigators, provided the mathematicians have adequately fulfilled their job. If, however, the navigators fail to follow the mathematics, or even to check the results by taking a few examples, then there will be plenty of misunderstandings; to bring ‘philosophy’ into the discussion is to return to the attitudes of the Middle Ages, and to obscure technical issues which are perfectly clear and simple.