If actual matter had no other properties save such as are implied in the functional form of $G_{\mu}^{\nu} - \frac{1}{2} g_{\mu}^{\nu}G$, it would, I think, be impossible to make measurements with it. The property which makes it serviceable for measurement is discontinuity (not necessarily in the strict sense, but discontinuity from the macroscopic standpoint, i.e.\ atomicity). So far our only attempt to employ \index{Atomicity}% the new-found matter for measuring intervals has been in the study of the dynamics of a particle in \SecRef{56}; we had there to assume that discrete particles \PageSep{147} exist and further that they have necessarily a symmetry of field; on this understanding we have completed the cycle for one of our most important test-bodies---the moving particle---the geodesic motion of which is used, especially in astronomy, for comparing intervals. But the theory of the use of matter for the purpose of measuring intervals will be taken up in a more general way at the beginning of the next chapter, and it will be seen how profoundly the existence of the complete cycle has determined that outlook on the world which we express in our formulation of the laws of mechanics. It is a feature of our attitude towards nature that we pay great regard to \index{Energy-tensor of matter!obtained by Hamiltonian differentiation}% \index{Principle!of least action}% \index{Stationary action, principle of}% that which is permanent; and for the same reason the creation of anything \index{Creation of the physical world}% in the midst of a region is signalised by us as more worthy of remark than its entry in the orthodox manner through the boundary. Thus when we consider how an invariant depends on the variables used to describe the world, we attach more importance to changes which result in creation than to changes which merely involve transfer from elsewhere. It is perhaps for this reason that the Hamiltonian derivative of an invariant gives a quantity \index{Hamiltonian derivative!creative aspect of}% of greater significance for us than, for example, the ordinary derivative. The Hamiltonian derivative has a creative quality, and thus stands out in our minds as an active agent working in the passive field of space-time. Unless this idiosyncrasy of our practical outlook is understood, the Hamiltonian method with its casting away of boundary integrals appears somewhat artificial; but it is actually the natural method of deriving physical quantities prominent in our survey of the world, because it is guided by those principles which have determined their prominence. The particular form of the Hamiltonian method known as Least Action, in which special search is made \index{Action, principle of Stationary}% for Hamiltonian derivatives which vanish, does not appear at present to admit of any very general application. In any case it seems better adapted to give neat mathematical formulae than to give physical insight; to grasp the