In 1787 the Principal of the University of St Andrews died and Leslie no longer felt obliged to follow the conditions of his scholarship which required him to join the Church of Scotland. Instead he decided that he wanted to teach mathematics and Adam Smith (the philosopher and political economist) obtained a position for Leslie as a tutor to one of his relations.
Sa 1787 ang Principal ng University of St Andrews namatay at Leslie hindi na madama nagpapasalamat na sundin ang mga kondisyon ng kanyang karunungan na kinakailangan sa kanya upang sumali sa Simbahan ng Scotland. Sa halip siya ay nagpasya na niya kinakailangan na magturo matematika at Adam Smith (ang pilosopo at pampulitikang ekonomista) na nakuha ng isang posisyon para sa Leslie bilang isang propesor sa isa sa kanyang relasyon.