A person has to study until he learns to discriminate between the physical self and the rest of his being.
For the distinction between matter and spirit is not a natural division.
It emerges as a result of observation and introspection.
Thus, physical desire, whether for the good or otherwise, is at first grasped by man as the desire of his very self.
Only afterward is there room for discrimination:
What do I really want?
What is merely a reflex action and what is an inner need?
–Rabbi Adin Steinsaltz
From The Sustaining Utterance, p. 3, by Rabbi Adin Steinsaltz