cheaper by requiring less; consequently, there is nothing which, upon Mr. Ricardo's theory, can long continue stationary in value. If, therefore, he had proposed any measure of value, he must have forgotten his own principle of value. _Phil_. But allow me to ask; if that principle is not proposed as a measure of value, in what character _is_ it proposed? _X_. Surely, Philebus, as the _ground_ of value; whereas a measure of value is no more than a _criterion_ or test of value. The last is simply a _principium cognoscendi_, whereas the other is a _principium essendi_. _Phil_. But wherein lies the difference? _X_. Is it possible that you can ask such a question? A thermometer measures the temperature of the air; that is, it furnishes a criterion for ascertaining its varying degrees of heat; but you cannot even imagine that a thermometer furnishes any _ground_ of this heat. I wish to know whether a day's labor at the time of the English Revolution bore the same value as a hundred years after at the time of the French Revolution; and, if not the same value, whether a higher or a lower. For this purpose, if I believe that there is any commodity which is immutable in value, I shall naturally compare a day's labor with that commodity at each period. Some, for instance, have imagined that corn is of invariable value; and, supposing one to adopt so false a notion, we should merely have to inquire what quantity of corn a day's labor would exchange for at each period, and we should then have determined the relations of value between labor at the two periods. In this case, I should have used corn as the _measure_ of the value of labor; but I could not rationally mean to say that corn was the _ground_ of the value of labor; and, if I said that I made use of corn to _determine_ the value of labor, I should employ the word "determine" in the same sense as when I say that the thermometer determines the heat--namely, that it ascertains it, or determines it to my knowledge (as a _principium cognoscendi_). But, when Mr. Ricardo says that the quantity of labor employed on A determines the value of A, he must of course be understood to mean that it _causes_ A to be of this value, that it is the _ground_ of its value, the _principium essendi_ of its value; just as when, being asked what determines a stone to fall downwards rather than upwards, I answer that it is the earth's attraction, or the principle of gravitation, meaning that this principle _causes_ it to fall downwards; and if, in this case, I say that gravitation "_determines_" its course downwards, I no longer use that word in the sense of _ascertain_; I do not mean that gravitation _ascertains_ it to have descended; but that gravitation has _causatively_ impressed that direction on its course; in other words, I make gravitation the _principium essendi_ of its descent.