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:

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