Take, first, the rigid determinism of traditional physics, which was to have been avoided. Spinoza rightly perceived that this could not be avoided by such methods, and therefore accepted determinism in the psychical as in the physical realm. If everything we _say_ is determined by physical causes, our thoughts are only free when we tell lies: so long as we say what we think, our thoughts also can be inferred from physics. The philosophy which I advocate escapes this consequence in several ways. In the first place, causality does not involve _compulsion_, but only a law of sequence: if physical and mental events run parallel, either may with equal justice be regarded as causing the other, and there is no sense in speaking of them as causally independent. Thus the Cartesian dualism does not have the pleasant consequences which were intended. In the second place, modern physics has become less deterministic than the physics of the past few centuries. We do not know, _e.g._ what makes a radio-active atom explode or an electron jump from a larger to a smaller orbit. In these matters we only know statistical averages. Take next the view that mind and matter are quite disparate. This we have criticised already. It rests upon a notion that we know much more about matter than we do, and in particular upon the belief that the space of physics can be identified with the space of sensible experience. This belief is absent in Leibniz, who, however, never quite realised what his own view was. It is not absent in Kant, who realised that the space of sensible experience is subjective, and inferred that the space of physics is subjective. Since Kant, no one seems to have thought clearly about space until Einstein and Minkowski. The separation of physical and sensible space, logically carried out, shows the groundlessness of traditional views about mind and matter. This part of Descartes’ philosophy, therefore, though it accelerated the progress of physics, must be regarded as metaphysically an aberration.