One noticeable thing in favor of the two schools is that their method is strictly scientific, in the modern sense of the term. It is a posteriori and not a priori; it is inductive. Yet however scientific their method may seem to be their conclusions cannot be accepted as final. For the inductive method, according to the great French scientist, Poincaré, cannot give us exact knowledge because its experiments do not cover all the instances in a given case. There can be only a partial verification. There will always remain some phenomena that cannot be brought within the sphere of a particular observation.[102] Now, this is exactly the case in the subject under consideration. Only in some phases does the Yezidi religion resemble that of the old Persians. There are other beliefs which do not come under this category, and which seem to bear the traces of some other religions. What are we to do with these?[103] The advocates of the theory in question admit that such is the case, but they assert that “the resemblances of the Yezidi religion to Christianity and Islam are accidental”; that “owing to the residence of the Yezidis among the Mohammedans, the sect naturally has much in common with Islam.”[104] But why are the resemblances to Iranism not to be accounted for in the same way as those to other religions? Why may not equally strong inference be made from the likeness to Christianity? And what is the basis of such a discrimination? On these questions we are left entirely in the dark. Now, it is this lack of ground for their method of procedure that leads one to seek the solution of the problem on some other verifiable hypothesis. There is still another school among the western orientalists. I refer to those who maintain that the Yezidi sect was founded by Šeiḫ ‘Adi. A modern writer who holds this theory, after critically reviewing the views held by the different scholars, proceeds to advance his own idea. To emphasize it, and leave no room for further criticism, he claims that the theory has been “generally” accepted. To quote: “It is generally agreed upon that the sect of the Yezidis was founded by Šeiḫ ‘Adi. He is a historical personage, but it is exceedingly difficult, and almost impossible, to establish any historical facts out of the mist of very fantastic stories current about him.”[105] He supports his notion by an appeal to an Arab author, Kasi Ahmad ibn-Ḫallikan, from whom, according to this writer, an extract relating to Šeiḫ ‘Adi was published by one who for years was a resident of the city of Mosul.[106] This statement that Ibn Ḫallikan gives the biography of ‘Adi is a fact that cannot be questioned; but that ‘Adi founded the Yezidi sect is a theory that is by no means “generally agreed upon.” Nor can it be substantiated. To justify this position, let me quote in full what the Arab biographer and two other Mohammedan scholars have to say on the problem. 1 What Ibn Ḫallikan has to say on Šeiḫ ‘Adi: