(Αἰνησίδημος), a celebrated sceptic, born at Cnossus, in Crete, according to Diogenes Laertius (9.116), but at Aegae, according to Photius (Phot. Bibl. 212), probably lived a little later than Cicero. He was a pupil of Heracleides and received from him the chair of philosophy, which had been handed down for above three hundred years from Pyrrhon, the founder of the sect. For a full account of the sceptical system see PYRRHON. As Aenesidemus differed on many points from the ordinary sceptic, it will be convenient before proceeding to his particular opinions, to give a short account of the system itself.

The sceptic began and ended in universal doubt. He was equally removed from the academic who denied, as from the dogmatic philosopher who affirmed; indeed, he attempted to confound both in one, and refute them by the same arguments. (Sext. Emp. 1.1.) Truth, he said, was not to be desired for its own sake, but for the sake of a certain repose of mind (ἀταραξία) which followed on it, an end which the septic best attained in another way, by suspending his judgment (ἐποχή), and allowing himself literally to rest in doubt. (1.4.) With this view he must travel over the whole range of moral, metaphysical, and physical science. His method is the comparison of opposites, and his sole aim to prove that nothing call be proved, or what he termed, the ἰσοσθένεια, of things. In common life he may act upon Φαινόμενα with the rest of men : nature, law, and custom are allowed to have their influence; only when impelled to any vehemlent effort we are to remember that, here too, there is much to be said on both sides, and are not to lose our peace of mind by grasping at a shadow.

The famous δέκα τρόποι of the sceptics were a number of heads of argument intended to overthrow truth in whatever form it might appear. [PYRRHON.] The opposite appearances of the moral and natural world (Sext. Emp. 1.14), the fallibility of intellect and sense, and the illusions produced upon them by intervals of time and space and by very change of position, were the first arguments by which they assailed the reality of things. We cannot explain what man is, we cannot explain what the senses are: still less do we know the way in which they are acted upon by the mind (2.4-7): beginning with οὐδέν ὁρίζω, we must end with οὐδὲν μᾶλλον We are not certain whether material objects are anything but ideas in the mind: at any rate the different qualities which we perceive in them may be wholly dependent on the percipient being; or, supposing them to contain quality as well as substance, it may be one quality varying with the perceptive power of the different senses. (2.14.) Having thus confounded the world without and the world within, it was a natural transition for the sceptic to confound physical and metaphysical argluments. The reasonings of natural philosophy were overthrown by metaphysical subtleties, and metaphysics made to look absurd by illustrations only applicable to material things. The acknowledged imperfection of language was also pressed into the service; words, they said, were ever varying in their signification, so that the ideas of which they were the signs must be alike variable. The leading idea of the whole system was, that all truth involved either a vicious circle or a petitio principii, for, even in the simplest truths, something must be assumed to make the reasoning applicable. The truth of the senses was known to us from the intellect, but the intellect operated through the senses, so that our knowledge of the nature of either depends upon the other. There was, however, a deeper side to this philosophy. Everything we know, confessedly, runs up into something we do not know: of the true nature of cause and effect we are ignorant, and hence to the favourite method, ἀπὸ τοῦ εἰς ἄπειρον ἐκβάλλειν, or arguing backward fiom cause to cause, the very imperfection of human faculties prevents our giving an answer. We must know what we believe; and how can we be sure of secondary causes, if the first cause be wholly beyond us? To judge, however, from the sketch of Sextus Empiricus (Pyrrh. Hyp.), it was not this side of their system which the sceptics chiefly urged: for the most part, it must be confessed, that they contented themselves with dialectic subtleties, which were at once too absurd for refutation, and impossible to refute.

The causes of scepticism are more fully given under the article PYRRHON. One of the most remarkable of its features was its connexion with the later philosophy of the Ionian school. From the failure of their attempts to explain the phenomena of the visible world, the Ionian philosophers were insensibly led on to deny the order and harmony of creation: they saw nothing but a perpetual and ever-changing chaos, acted upon, or rather self-acting, by an inherent power of motion, of which the nature was only known by its effects. This was the doctrine of Heracleitus, that "the world was a fire ever kindling and going out, which made all things and was all things." It was this link of connexion between the sceptical and Ionian schools which Aenesidemus attempted to restore. The doctrine of Heracleitus, although it spoke of a subtle fire, really meant nothing more than a principle of change; and although it might seem absurd to a strict sceptic like Sextus Empiricus to affirm even a principle of change, it involved no real inconsistency with the sceptical system. We are left to conjecture as to the way in which Aenesidemus arrived at his conclusions : the following account of them seems probable. It will be seen, from what has been said, that the sceptical system had destroyed everything but sensation. But sensation is the effect of change, the principle of motion working internally. It was very natural then that the sceptic, proceeding from the only ἀρχή, which remained to him, should suggest an explanation of the outward world, derived from that of which alone he was certain, his own internal sensations. The mere suggestion of a probable cause might seem inconsistent with the distinction which the sceptics drew between their own absolute uncertainty and the probability spoken of by the Academics indeed, it was inconsistent with their metaphysical paradoxes to draw conclusions at all : if so, we must be content to allow that Aenesidemus (as Sextus Empiricus implies) got a little beyond the dark region of scepticism into the light of probability.

Other scattered opinions of Aenesidemus have been preserved to us, some of which seem to lead to the same conclusion. Time, he said, was τὸ ὂν and τὸ πρῶτον σῶμα (Pyr. Hyp. 3.17), probably in allusion to the doctrine of the Stoics, that all really existing substances were σώματα: in other words, he meant to say that time was a really existing thing, and not merely a condition of thought. This was connected with the principle of change, which was inseparable from a notion of time : if the one had a real existence (and upon its existence the whole system depended), the other must likewise have a real existence. In another place, adapting his language to that of Heracleitus, he said that "time was air" (Sext. Emp. ad v. Logicos, 4.233.), probably meaning to illustrate it by the imperceptible nature of air, in the same way that the motion of the world was said to work by a subtle and invisible tire. All things, according to his doctrine, were but Φαινόμενα which were brought out and adapted to our perceptions by their mutual opposition: metaphorically they might be said to shine forth in the light of Heracieitus's fire. He did not, indeed, explain how this union of opposites made them sensible to the faculties of man : probably he would rather have supported his view by the impossibility of the mind conceiving of anything otherwise than in a state of motion, or, as he would have expressed it, in a state of mutual opposition. But Φαινόμενα are of two kinds, ἴὸια and κοινὰ (Sext. Emp. ad v. Log. 2.8), the perceptions of individuals, and those common to mankind. Here again Aenesidemus seems to lose sight of the sceptical system. which (in speculation at least) admitted no degrees of truth, doubt, or probability. The same remark applies to his distinction of κίνησις into μεταβατική and μεταβλητική, simple motion and change. He seems also to have opposed the perplexity which the sceptics endeavoured to bring about between matter and mind; for he asserted that thought was independent of the body, and "that the sentient power looked out through the crannies of the senses." (Adv. Log. 1.349.) Lastly, his vigorous mind was above the paltry confusion of physical and metaphysical distinctions; for he declared, after Heracleitus, "that a part was the same with the whole and yet different from it." The grand peculiarity of his system was the attempt to unite scepticism with the earlier philosophy, to raise a positive foundation for it by accounting from the nature of things for the never-ceasing changes both in the material and spiritual world.

Sextus Empiricus has preserved his argument against our knowledge of causes, as well as a table of eight methods by which all a priori reasoning may be confuted, as all arguments whatever may be by the δέκα τρόποι. I. Either the cause given is unseen, and not proven by things seen, as if a person were to explain the motions of the planets by the music of the spheres. II. Or if the cause be seen, it cannot be shewn to exclude other hypotheses: we must not only prove the cause, but dispose of every other cause. III. A regular effect may be attributed to an irregular cause ; as if one were to explain the motions of the heavenly bodies by a sudden impulse. IV. Men argue from things seen to things unseen, assuming that they are governed by the same laws. V. Causes only mean opinions of causes, which are inconsistent with phenomena and with other opinions. VI. Equally probable causes are accepted or rejected as they agree with this or that preconceived notion. VII. These causes are at variance with phenomena as well as with abstract principles. VIII. Principles must be uncertain, because the facts from which they proceed are uncertain. (Pyrrh. Hyp. 1.17, ed. Fabr.)

It is to be regretted that nothing is known of the personal history of Aenesidemus. A list of his works and a sketch of their contents have been preserved by Photius. (Cod. 212.) He was the author of three books of Πυρ̀ῥώνειαι Ὑποτυπώσεις, and is mentioned as a recent teacher of philosophy by Aristocles. (Apud Euscb. Praeparat. Exang. 14.18.) It is to Aenesidemus that Sextus Empiricus was indebted for a considerable part of his work.