From these reasonings the Academics taught that nothing was certain, nothing was to be known ([Greek: katalepton]). For the Stoics themselves, their most determined opponents, defined the [Greek: kataleptike phantasia] (the phantasy or impression which involved knowledge[160a]) to be one that was capable of being produced by no object except that to which it really belonged.[161] Since then we cannot arrive at knowledge, we must suspend our decision, pronounce absolutely on nothing, nay, according to Arcesilas, never even form an opinion.[162] In the conduct of life, however, probability[163] must determine our choice of action; and this admits of different degrees. The lowest kind is that which suggests itself on the first view of the case ([Greek: phantasia pithane], or _persuasive phantasy_); but in all important matters we must correct the evidence of our senses by considerations derived from the nature of the medium, the distance of the object, the disposition of the organ, the time, the manner, and other attendant circumstances. When the impression has been thus minutely considered, the _phantasy_ becomes [Greek: aperiodeumene], or _approved on circumspection_; and if during this examination no objection has arisen to weaken our belief, the highest degree of probability is attained, and the phantasy is pronounced _unembarrassed with doubt_, or [Greek: aperispastos].[164] Sextus Empiricus illustrates this as follows:[165] If on entering a dark room we discern a coiled rope, our first impression may be that it is a serpent--this is the _persuasive phantasy_. On a closer inspection, however, after _walking round it_ ([Greek: periodeusantes]), or _on circumspection_, we observe it does not move, nor has it the proper colour, shape, or proportions; and now we conclude it is not a serpent; here we are determined in our belief by the [Greek: periodeumene phantasia], and we assent to the _circumspective phantasy_. For an instance of the third and most accurate kind, viz., that with which no contrary impression interferes, we may refer to the conduct of Admetus on the return of Alcestis from the infernal regions. He believes he sees his wife; everything confirms it; but he cannot simply acquiesce in that opinion, because his mind is _embarrassed or distracted_ [Greek: perispatai] from the knowledge he has of her having died; he asks, "What! do I see my wife I just now buried?" (_Alc._ 1148.) Hercules resolves his difficulty, and his phantasy is in repose, or [Greek: aperispastos]. The suspension then of assent ([Greek: epoche]) which the Academics enjoined, was, at least from the time of Carneades,[166] almost a speculative doctrine;[167] and herein lay the chief difference between them and the Pyrrhonists; that the latter altogether denied the existence of the probable, while the former admitted there was sufficient to allow of action, provided we pronounced absolutely on nothing.