“There are seven causes of inconsistencies and contradictions to be met with in a literary work. The first cause arises from the fact that the author collects the opinions of various men, each differing from the other, but neglects to mention the name of the author of any particular opinion. In such a work contradictions or inconsistencies must occur, since any two statements may belong to two different authors. Second cause: The author holds at first one opinion which he subsequently rejects: in his work, however, both his original and altered views are retained. Third cause: The passages in question are not all to be taken literally: some only are to be understood in their literal sense, while in others figurative language is employed, which includes another meaning besides the literal one: or, in the apparently inconsistent passages, figurative language is employed which, if taken literally, would seem to be contradictories or contraries. Fourth cause: The premises are not identical in both statements, but for certain reasons they are not fully stated in these passages: or two propositions with different subjects which are expressed by the same term without having the difference in meaning pointed out, occur in two passages. The contradiction is therefore only apparent, but there is no contradiction in reality. The fifth cause is traceable to the use of a certain method adopted in teaching and expounding profound problems. Namely, a difficult and obscure theorem must sometimes be mentioned and assumed as known, for the illustration of some elementary and intelligible subject which must be taught beforehand the commencement being always made with the easier thing. The teacher must therefore facilitate, in any manner which he can devise, the explanation of those theorems, which have to be assumed as known, and he must content himself with giving a general though somewhat inaccurate notion on the subject. It is, for the present, explained according to the capacity of the students, that they may comprehend it as far as they are required to understand the subject. Later on, the same subject is thoroughly treated and fully developed in its right place. Sixth cause: The contradiction is not apparent, and only becomes evident through a series of premises. The larger the number of premises necessary to prove the contradiction between the two conclusions, the greater is the chance that it will escape detection, and that the author will not perceive his own inconsistency. Only when from each conclusion, by means of suitable premises, an inference is made, and from the enunciation thus inferred, by means of proper arguments, other conclusions are formed, and after that process has been repeated many times, then it becomes clear that the original conclusions are contradictories or contraries. Even able writers are liable to overlook such inconsistencies. If, however, the contradiction between the original statements can at once be discovered, and the author, while writing the second, does not think of the first, he evinces a greater deficiency, and his words deserve no notice whatever. Seventh cause: It is sometimes necessary to introduce such metaphysical matter as may partly be disclosed, but must partly be concealed: while, therefore, on one occasion the object which the author has in view may demand that the metaphysical problem be treated as solved in one way, it may be convenient on another occasion to treat it as solved in the opposite way. The author must endeavour, by concealing the fact as much as possible, to prevent the uneducated reader from perceiving the contradiction.”

Guide for the Perplexed (c. 1190), Introduction