✗ A doctor can’t operate if he doesn’t know the reason of the illness.
✓ A doctor can’t operate if he doesn’t know the reason for the illness.
Other words meaning reason
cause the reason that something happens or that you feel a particular emotion: The probable cause of death was drowning. ♦ Fifty years after the war, scholars still disagree about its causes.
excuse a reason you give in order to explain why you did something bad or wrong: She gave some excuse about being too sick to finish her essay.
explanation a fact or set of facts that tells you why something happened: There was no obvious explanation for his sudden disappearance.
grounds a word used in official or legal situations, meaning a good or fair reason for doing something: His repeated violence towards her was given as grounds for divorce. ♦ Permission for the march was refused, on grounds of public safety.
motivation someone’s personal reason for doing something: The other runners’ times were fast, and that gave me motivation to push even harder.
motive someone’s personal reason for doing something, especially something dishonest or illegal: Police are unsure about a motive for the crime.
pretext a false reason you give for doing something in order to hide your real reason: He got into the warehouse on the pretext of making a building inspection.
purpose the goal that you want to achieve: The purpose of Tuesday’s meeting is to finalize the schedule.