Since Aristotle had a better apprehension of the theory of value than other Greek thinkers, we may expect him also to define more clearly the concept of wealth. In the _Politics_, he names the following attributes of genuine (ἀληθινός) wealth (πλοῦτος): necessary to life; useful to persons associated in a household or a state; capable of accumulation (θησαυρισμός); limited in extent (οὐκ ἄπειρος).[543] According to Mill,[544] from the “economic” standpoint, wealth is “all useful and agreeable things” of a “material nature” possessing “exchange value”; and, to have exchange value, they must be “capable of accumulation.” In comparing these two definitions, it should be recognized at the outset that Aristotle’s term “genuine” does not mean “truly economic,” as it might in Mill, but rather “legitimate wealth” as distinguished from that gained from false finance (χρηματιστική);[545] also that his “necessary to life” and “limited in extent” are not used in the economic but in the moral sense, as opposed to luxury and extreme interest in money-making. Mill’s “all useful and agreeable things” presents a marked contrast to this in spirit. Aristotle’s “useful” means “what subserves the final good” (πρὸς ἀγαθὴν ζωήν), while Mill’s means “things that give sensations of comfort or pleasure.” Thus Aristotle’s wealth is necessarily limited, while Mill’s is unlimited, since, as Barker observes, “only an infinity of wealth can satisfy an infinity of need.”[546] It will be seen from the following discussion, however, that Aristotle includes more than “necessary things” in his category of economic wealth. He does not specify “material things,” as does Mill, but it seems probable that this is his meaning.[547] In all the passages where he enumerates the different kinds of wealth, only material things are included, except slaves, who are counted as mere tools.[548] One of these passages specifically excludes intellectual wealth by defining property as a “separable instrument.”[549] The use of the term for value (ἀξία) probably implies the same limitation.[550] Though Aristotle does not mention exchange value specifically, it is clearly implied in his definition. “Things useful for the association of a state” and things “capable of accumulation” must have exchange value, thus excluding illimitable utilities such as air and light.[551] His use of κτῆμα, “possession,” and his recognition of cost of production and economic demand as the main factors in determining value,[552] are further evidence of this. Moreover, as seen above, in the _Ethics_, he clearly makes exchange value an attribute of all wealth.[553]