The Talmud on Observing the Fast

[If a person is] ill [and requires food due to potential danger], one feeds him according to [the advice of medical] experts. R. Yannai said: [If] an ill person says [he] needs [to eat], and a doctor says he does not need [to eat], one listens to the ill person.

What is the reason [for this halakhah? It is because the verse states]: The heart knows the bitterness of its soul (Proverbs 14:10), [meaning an ill person knows the intensity of his pain and weakness, and doctors cannot say otherwise. The Gemara asks: It is] obvious [that a person knows himself better than anyone else does. Why does this need to be stated explicitly? The Gemara answers: It is] lest you say [that] the doctor is more certain [because he has had more experience with this condition. Therefore, the verse] teaches us [that even so, it is the ill person who knows his own suffering better than anyone else].

[However, in the opposite case, if] a doctor says [that the ill person] needs [food], but the ill person [himself] says he does not need [to eat], one listens to the doctor. What is the reason [for this halakhah? It is because] confusion has taken hold [of the ill person on account of his illness, and his judgment is impaired. Consequently, he himself does not know how much he needs food].