Yet one need merely consider the various propositions that come forth at the outset of proper (empirical) physics, such as those of the persistence of the same quantity of matter, of inertia, of the equality of effect and counter effect, etc., and one will quickly be convinced that they constitute a physica pura (or rationalis), which well deserves to be separately established, as a science of its own, in its wholedomain, whether narrow or wide.