I. Since aetiology in general refers to things that are unseen, it does not give testimony that is incontestable in regard to phenomena.  For example, the Pythagoreans explain the distance of the planets by a musical proportion.

II.  From many equally plausible reasons which might be given for the same thing, one only is arbitrarily chosen, as some explain the inundation of the Nile by a fall of snow at its source, while there could be other causes, as rain, or wind, or the action of the sun.

III.  Things take place in an orderly manner, but the causes presented do not show any order, as for example, the motion of the stars is explained by their mutual pressure, which does not take into account the order that reigns among them.

IV.  The unseen things are supposed to take place in the same way as phenomena, as vision is explained in the same way as the appearance of images in a dark room.

V. Most philosophers present theories of aetiology which agree with their own individual hypotheses about the elements, but not with common and accepted ideas, as to explain the world by atoms like Epicurus, by homoeomeriae like Anaxagoras, or by matter and form like Aristotle.

VI.  Theories are accepted which agree with individual hypotheses, and others equally probable are passed by, as Aristotle’s explanation of comets, that they are a collection of vapors near the earth, because that coincided with his theory of the universe.

VII.  Theories of aetiology are presented which conflict not only with individual hypotheses, but also with phenomena, as to admit like Epicurus an inclination or desire of the soul, which was incompatible with the necessity which he advocated.

VIII.  The inscrutable is explained by things equally inscrutable, as the rising of sap in plants is explained by the attraction of a sponge for water, a fact contested by some.[2]

    [1] Hyp. I. 98.

    [2] Hyp. I. 180-186; Fabricius, Cap.  XVII. 180 z.

Diogenes does not mention these Tropes in this form, but he gives a resume of the general arguments of the Sceptics against aetiology,[1] which has less in common with the eight Tropes of Aenesidemus, than with the presentation of the subject by Sextus later,[2] when he multiplies his proofs exceedingly to show [Greek:  meden einai aition].  Although the Tropes of Aenesidemus have a dialectic rather than an objective character, it would not seem that he made the distinction, which is so prominent with Sextus, between the signs [Greek:  hypomnestika] and [Greek:  endeiktika],[3] especially as Diogenes sums up his argument on the subject with the general assertion, [Greek:  Semeion ouk einai],[4] and proceeds to introduce the logical consequence of the denial of aetiology.  The summing up of the Tropes of Aenesidemus is given as follows, in the Hypotyposes, by Sextus:—­“A cause in harmony with all the sects of philosophy, and with Scepticism, and with phenomena, is perhaps not possible, for the phenomena and the unknown altogether disagree."[5]