It is important to contrast value as thus conceived, in its formal and logical aspects, with other historical conceptions of value. In the classification which follows, the writer has by no means attempted an exhaustive list. Definitions of value are very numerous, but it is not necessary to list them all, since many differ, not so much in their logical or formal aspects, as in the theory of the origin of value which the definition is made to include. There are two principles of classification which will be used, however, which, used in a cross-classification, will enable us to exhibit the contrasts of most importance for present purposes. The first line of cleavage is between the conceptions which treat value as an ethical ideal, often different from the market fact, and those which accept the value which is expressed in prices in the market as the "real or true" value for economic science. The medieval conception of the _justum pretium_ belongs to the first class, as does also the conception of President Hadley: "The price of an article or service, in the ordinary commercial sense, is the amount of money which is paid, asked, or offered for it. The value of an article or service, is the amount of money which may properly be paid, asked, or offered for it."[17] And the value theory of Karl Marx, though differing from either of these in points, is yet like them in this one respect: value and price do not necessarily agree for Marx. The value of a thing for him depends on the "socially necessary" labor embodied in it, while some things, as land, command a price in the market, even though embodying no labor.[18] Opposed to this group of theories are, doubtless, the greater part of present-day writers, who, while differing among themselves at many points, would insist that value is a fact, and not an ideal. The second line of division is between the conceptions of value as a quantity and value as a ratio, or, to put the thing more generally and more accurately, between the value of a thing as a definite magnitude, independent of exchange relations, and that value as a relative thing, not only _measured_ by the process of exchanging, but also caused by it, and varying with the value of the things with which the article is compared. Professor Clark and his followers belong in the second group of the first classification, and in the first group of the second classification. The social value of which they speak is a fact, and not an ideal (though Professor Clark has often been interpreted as teaching that the fact corresponds closely with an ideal), and social value as treated by them (noting the exception of Professor Seligman, who does not follow Professor Clark closely), is an absolute magnitude.[19] Karl Marx and Henry George agree with them upon this latter point. Value is a quantity, and not a mere relation, for both.[20] Wieser would concur here.[21]