Lecture I

I have reminded you of all this, in order clearly to describe the point at which we pass, from what can still be determined “a priori”, into the realm of real Nature. We need to be very clear on this point. The truths of arithmetic, geometry and kinematics,—these we undoubtedly determine apart from external Nature. But we must also be clear, to what extent these truths are applicable to that which meets us, in effect, from quite another side—and, to begin with, in mechanics. Not till we get to mechanics, have we the content of what we call “phenomenon of Nature”.