Hello David. Isnt it considered external obligation because of outer circumstances..The conditions necessitate that ,so we can use (has to)
Hello, Ihab—"Has to" would work if the speaker were the one who was ill and were compelling himself (intellectually) to see a doctor on that basis.
I am ill. I think I have to see a doctor.
However, "has to" doesn't work in your sentence, at least in a normal context. Illness does not force a person to see a doctor. For example, millions of ill people in the United States avoid going to the doctor because they don't have health insurance and don't want to incur many thousands of dollars in medical bills. Going to the hospital to see a doctor could put them in debt for years.
In the following special context, "has to" would work. One could say, "He is ill. I think he has to see a doctor" if one believed that the reason he was ill was that he had to see a doctor. Some people dread going to see a doctor because of illness, as, for example, those who think it might drain their life savings. For such people, needing to go to the doctor could of itself induce illness.
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