Another doctrine, respecting knowledge and definition, is ascribed by Aristotle to "the disciples of Antisthenes and other such uninstructed persons": it is also canvassed by Plato in the Theætêtus,[137] without specifying its author, yet probably having Antisthenes in view. As far as we can make out a doctrine which both these authors recite as opponents, briefly and their own way, it is as follows:--"Objects must be distinguished into--1. Simple or primary; and 2. Compound or secondary combinations of these simple elements. This last class, the compounds, may be explained or defined, because you can enumerate the component elements. By such analysis, and by the definition founded thereupon, you really come to _know_ them--describe them--predicate about them. But the first class, the simple or primary objects, can only be perceived by sense and named: they cannot be analysed, defined, or known. You can only predicate about them that they are like such and such other things: _e.g., silver_, you cannot say what it is in itself, but only that it is like tin, or like something else. There may thus be a _ratio_ and a definition of any compound object, whether it be an object of perception or of conception: because one of the component elements will serve as Matter or Subject of the proposition, and the other as Form or Predicate. But there can be no definition of any one of the component elements separately taken: because there is neither Matter nor Form to become the Subject and Predicate of a defining proposition." [Footnote 137: Plato, Theætêt, pp. 201-202. Aristotel. Metaph. [Greek: Ê]. 1043, b. 22.] This opinion, ascribed to the followers of Antisthenes, is not in harmony with the opinion ascribed by Aristotle to Antisthenes himself (_viz._, That no propositions, except identical propositions, were admissible): and we are led to suspect that the first opinion must have been understood or qualified by its author in some manner not now determinable. But the second opinion, drawing a marked logical distinction between simple and complex Objects, has some interest from the criticisms of Plato and Aristotle: both of whom select, for the example illustrating the opinion, the syllable as the compound made up of two or more letters which are its simple constituent elements. [Remarks of Plato on this doctrine.]