[1] Saisset Op. cit. p. 237.

    [2] Hyp. I. 178.

    [3] Zeller III. 38; Ritter IV. 277.

    [4] Saisset Op. cit. p. 231.

The two Tropes are founded on the principle that anything must be known through itself or through something else.  It cannot be known through itself, because of the discord existing between all things of the senses and intellect, nor can it be known through something else, as then either the regressus in infinitum or the circulus in probando follow.[1] Diogenes Laertius does not refer to these two Tropes.

In regard to all these Tropes of the suspension of judgment, Sextus has well remarked in his introduction to them, that they are included in the eighth, or that of relation.[2]

    [1] Hyp. I. 178-179.

    [2] Hyp. I. 39.

The Tropes of Aetiology.  The eight Tropes against causality belong chronologically before the five Tropes of Agrippa, in the history of the development of sceptical thought.  They have a much closer connection with the spirit of Scepticism than the Tropes of Agrippa, including, as they do, the fundamental thought of Pyrrhonism, i.e., that the phenomena do not reveal the unknown.

The Sceptics did not deny the phenomena, but they denied that the phenomena are signs capable of being interpreted, or of revealing the reality of causes.  It is impossible by a research of the signs to find out the unknown, or the explanation of things, as the Stoics and Epicureans claim.  The theory of Aenesidemus which lies at the foundation of his eight Tropes against aetiology, is given to us by Photius as follows:[1] “There are no visible signs of the unknown, and those who believe in its existence are the victims of a vain illusion.”  This statement of Aenesidemus is confirmed by a fuller explanation of it given later on by Sextus.[2] If phenomena are not signs of the unknown there is no causality, and a refutation of causality is a proof of the impossibility of science, as all science is the science of causes, the power of studying causes from effects, or as Sextus calls them, phenomena.

It is very noticeable to any one who reads the refutation of causality by Aenesidemus, as given by Sextus,[3] that there is no reference to the strongest argument of modern Scepticism, since the time of Hume, against causality, namely that the origin of the idea of causality cannot be so accounted for as to justify our relying upon it as a form of cognition.[4]

    [1] Myriob. 170 B. 12.

    [2] Adv.  Math. VIII. 207.

    [3] Hyp. I. 180-186.

    [4] Ueberweg Op. cit. p. 217.

The eight Tropes are directed against the possibility of knowledge of nature, which Aenesidemus contested against in all his Tropes, the ten as well as the eight.[1] They are written from a materialistic standpoint.  These Tropes are given with illustrations by Fabricius as follows: