Distance, parallax and cubic parallax have the same kind of potential existence even when the operations of measurement are not actually made---\emph{if} you will move sideways you will be able to determine the angular shift, \emph{if} you will lay measuring-rods in a line to the object you will be able to count their number. Any one of the three is an indication to us of some existent condition or relation in the world outside us---a condition not created by our operations. But there seems no reason to conclude that this world-condition \emph{resembles} distance any more closely than it resembles parallax or cubic parallax. Indeed any notion of ``resemblance'' between physical quantities and the world-conditions underlying them seems to be inappropriate. If the length~$AB$ is double the length~$CD$, the parallax of~$B$ from~$A$ is half the parallax of~$D$ from~$C$; there is undoubtedly some world-relation which is different for $AB$ and~$CD$, but there is no reason to regard the world-relation of~$AB$ as being better represented by double than by half the world-relation of~$CD$. The connection of manufactured physical quantities with the existent world-condition can be expressed by saying that the physical quantities are \emph{measure-numbers} of the world-condition. Measure-numbers may be assigned \index{Measure-code}% according to any code, the only requirement being that the same measure-number always indicates the same world-condition and that different world-conditions receive different measure-numbers. Two or more physical quantities may thus be measure-numbers of the same world-condition, \emph{but in different codes}, e.g.\ parallax and distance; mass and energy; stellar magnitude and luminosity. The constant formulae connecting these pairs of physical quantities give the relation between the respective codes. But in admitting that physical quantities can be used as measure-numbers of world-conditions existing independently of our operations, we do not alter their status as manufactured quantities. The same series of operations will naturally manufacture the \PageSep{3} same result when world-conditions are the same, and different results when they are different. (Differences of world-conditions which do not influence the results of experiment and observation are \Foreign{ipso facto} excluded from the domain of physical knowledge.) The size to which a crystal grows may be a measure-number of the temperature of the mother-liquor; but it is none the less a manufactured size, and we do not conclude that the true nature of size is caloric.