[2] Adv.  Math. VII. 345.

    [3] Hyp. I. 38.

    [4] Diog.  IX. 11, 87.

    [5] Diog.  IX. 11, 87.

Pappenheim tries to prove[1] that Aenesidemus originally gave only nine Tropes in his Pyrrhonean Hypotyposes, as Aristocles mentions only nine in referring to the Tropes of Aenesidemus, and that the tenth was added later.  Had this been the case, however, the fact would surely have been mentioned either by Diogenes or Sextus, who both refer to the ten Tropes of Aenesidemus.

The Tropes claim to prove that the character of phenomena is so relative and changeable, that certain knowledge cannot be based upon them, and as we have shown, there is no other criterion of knowledge for the Sceptic than phenomena.[2] All of the Tropes, except the tenth, are connected with sense-perception, and relate to the difference of the results obtained through the senses under different circumstances.  They may be divided into two classes, i.e., those based upon differences of our physical organism, and those based upon external differences.  To the first class belong the first, second, third and fourth; to the second class, the fifth, sixth, seventh and eighth, and also the ninth.  The eighth, or that of relation, is applied objectively both by Sextus and Diogenes in their treatment of the Tropes, and is not used for objects of thought alone, but principally to show the relation of outward objects to each other.  The tenth is the only one which has a moral significance, and it has also a higher subjective value than the others; it takes its arguments from an entirely different sphere of thought, and deals with metaphysical and religious contradictions in opinion, and with the question of good and evil.  That this Trope is one of the oldest, we know from its distinct mention in connection with the foundation theories of Pyrrho, by Diogenes.[3] In treating of the subjective reasons for doubt as to the character of external reality, the Sceptics were very near the denial of all outward reality, a point, however, which they never quite reached.

    [1] Pappenheim, Die Tropen der Griechen, p. 23.

    [2] Hyp. I. 22.

    [3] Diog.  IX. 11, 61.

There is evidently much of Sextus’ own thought mixed with the illustrations of the Tropes, but it is impossible to separate the original parts from the material that was the common property of the Sceptical School.  Many of these illustrations show, however, perfect familiarity with the scientific and medical teachings of the time.  Before entering upon his exposition of the Tropes, Sextus gives them in the short concise form in which they must first have existed[1]—­

   (i) Based upon the variety of animals.

  (ii) Based upon the differences between men.

 (iii) Based upon differences in the constitution of
       the sense organs.

  (iv) Based upon circumstances.