The mechanical view of physical processes is thus an indispensable postulate of the various empirical sciences which seek to describe those processes by the aid of general formulæ. Hence the protests which are sometimes urged against the use of mechanical interpretations in descriptive science are really in spirit no more than the expression of a personal distaste for the whole business of scientific generalisation and description. If there are to be sciences of physical processes at all, these sciences must be mechanical, in the wider acceptation of the term. It does not, however, follow because the mechanical view of physical processes is a necessity for our empirical sciences, that this view is consequently ultimately true. As we have learned already, when we pass from the statement that the processes of the physical order may, for the purpose of description by general formulæ, and the invention of practical methods for their production, be treated as to all intents mechanical, to the very different assertion that the physical order really is rigidly mechanical, we have deserted empirical science for dogmatic Metaphysics, and our metaphysical dogma must stand or fall by its own ultimate coherency and intelligibility as a way of thinking about Reality. The usefulness of the mechanical interpretation for other purposes is no evidence whatever of its value for the special purpose of the metaphysician.[139] § 6. Our previous discussion has already satisfied us that, as Metaphysics, the postulate of Uniformity upon which the mechanical view of the physical order rests, is unintelligible and therefore indefensible. But we may supplement the discussion by one or two reflections which throw into striking relief the inadequacy of that concept of the physical order as a huge self-acting machine which is so often offered us to-day as the last word of scientific thought. In the mechanical metaphysical theories two points always receive special emphasis. The physical order, according to the thorough-going exponents of the doctrine, is a mechanism which is (_a_) self-contained and self-acting, and (_b_) entirely devoid of internal purpose.