replace the abstract frame in the mathematician's note-book, because as we have seen in~\Eq{(19.1)} the actual transformation of coordinates \emph{resulting} in a change of~$dx_{\mu}$ is the same as the transformation associated with the change of~$dx_{\mu}$ according to the law of a contravariant vector. I do not think it is too extravagant to claim that the method of the tensor \index{Tensor equations}% calculus, which presents all physical equations in a form independent of the choice of measure-code, is the only possible means of studying the conditions of the world which are at the basis of physical phenomena. The physicist is accustomed to insist (sometimes quite unnecessarily) that all equations should be stated in a form independent of the units employed. Whether this is desirable depends on the purpose of the formulae. But whatever additional insight into underlying causes is gained by stating equations in a form independent of units, must be gained to a far greater degree by stating them in a form altogether independent of the measure-code. An equation of this general form is called a \emph{tensor equation}. When the physicist is attacking the everyday problems of his subject, he may use any form of the equations---any specialised measure-plan---which will shorten the labour of calculation; for in these problems he is concerned with the outward significance rather than the inward significance of his \PageSep{50} formulae. But once in a while he turns to consider their inward significance---to consider that relation of things in the world-structure which is the origin of his formulae. The only intelligible idea we can form of such a structural relation is that it exists between the world-conditions themselves and not between the measure-numbers of a particular code. A law of nature resolves itself into a constant relation, or even an identity, of the two world-conditions to which the different classes of observed quantities forming the two sides of the equation are traceable. Such a constant relation independent of measure-code is only to be expressed by a tensor equation.