Abu Ja‘far medicine . . ."says: "Our belief concerning the reports on ash-Shaykh al-Mufid, may Allah have mercy upon him, says: Medicine is a proper science, and a discipline approved by revelation; the scholars took their knowledge of it from the prophets. For there is no means of diagnosing illness except by textual proof, and no means of telling the remedy except by what is prescribed. Thus, it has been established that the way to know this is by textual proof (sam‘) from Him Who knows the secret essence of things, or by reports which have been handed down on the authority of the truthful Imams, exemplified in the saying of the Commander of the Believers, "The stomach is the seat of every disease, and diet is the principal medicine, so let everyone keep his body to what he is accustomed to".
Also, a remedy which might prove useful to the people of one district might prove fatal to the people of another, and a medicine might benefit a people of a certain habit which would not help those of another. The truthful Imams would prescribe for one of the sick people a medicine which proved to be harmful to others suffering from the same complaint, without harming him, since they were aware that the cause of the illness (in the first case) had ceased; so he who uses this medicine, uses it after recovery without being conscious of this.
In fact, their knowledge of this was inspired by Allah, the Almighty, in the manner of a miracle and as a supernatural proof of their distinctive status. Thus, some people presumed that when the medicine combined with the germs of the illness, it would benefit them; and this they were greatly mistaken and injured others. However, this is a consideration which Abu Ja‘far has not mentioned, though it is relevant to this chapter. As for the interpretations put forward by him, they are quite sound; the traditions may bear the meaning which he mentioned.