Sextus Empiricus presented the sceptical arguments clearly, with special reference to Stoic materialism. Since later scepticism up to Hume's day went no further, it may be well to summarize the ten characteristic ways of procedure (tropes) formulated, possibly, with Aristotle's list of categories in mind. Desirous of disabling assent to any dogmatic view Sextus, following Aeneside mus, tries to show why a criterion of truth is impossible. It is to be remembered that the common effort of the dogmatic schools was to bestow the boon of independent tranquillity upon men. To this end they presumed some knowledge of the nature of the world, which played the part of an external support. Thus, for example, the Stoics taught that sensations produce modifications within the mind, and these, in turn, represent or symbolize ob jects. The presentations ("phantasies" or images) are "affections occurring in the soul, revealing both themselves and that which caused them." The sceptics held such doctrines to be unneces sary and impossible. Sensations or ideas, being states of indi vidual subjectivity, men can know nothing save their own inward experiences. Hence the intent of the "tropes." (I) Animals differ in their modes of perception, because they differ in the development of their senses. (2) Men similarly differ from each other. (3) The sense-organs differ, so that objects perceived by this or that sense present contrasted aspects. Did we possess more senses, unknown aspects of objects might appear. (4) The moods of precipients differ, and make it impossible to discern which affords true information about objects. (5) Objects differ
according to their distance and position—the stick seems bent when thrust into water. (6) The organs of sense-perception con stitute a medium through which the object must be perceived; hence objects cannot be known directly. (7) The modes of ob jects, never the objects themselves, appear to us. (8) Every thing in the external world is relative, not merely to the percipi ent, but to everything else. (9) Associations are of such a nature that we see things in false perspective, stressing the unfamiliar, for instance, and accepting the familiar without question. (io) Consideration of mankind shows that customs, opinions, man ners, institutions, laws and beliefs are of the most diverse ; even in the essential matter of right and wrong, no universal standard exists. Allowing that such qualities as truth, capacity to appeal to sensibility, and intelligibility are intrinsic to objects, it follows that "(I) nothing is self-evident, for if things were certain of themselves men would not differ about them; (2) nor can any thing be made certain by proof ; because we must either arrive in the process at something self-evident or involve ourselves in an endless regress." Accordingly, it is out of the question to argue from "signs" to things signified, and cause as an effective component of phenomena disappears. To be beyond reach of fear or doubt or other disturbance, a man must retire into him. self. Such is the theory. For the rest, he might (often did) acqui esce practically in the Roman conviction of "manifest destiny," to which Carneades would have allowed a measure of probability.