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: 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.