SPIRIT is an Intelligent Tutoring System for tutoring probability theory which has evolved through a continuous process of experimentation and tuning. The system manages a unique flexible tutoring style. On one hand, the system may behave as a tutor who mostly observes the student without interference, intervening only when things are really going wrong and on the other hand, it may behave as a tutor who manages a "questioning and answering" type of dialogue. Based on a belief constructed about the student's aptitude, the system frequently changes its tutoring style. SPIRIT integrates several artificial intelligence methods that include: a theorem prover; a production system; an object oriented system and procedural knowledge embedded in LISP code. (Author)