_A posteriori,_ however, definite facts are appealed to against freedom, some physical, others psychological. Sometimes it is asserted that our actions are necessitated by our feelings, our ideas, and the whole preceding series of our conscious states; sometimes freedom is denounced as being incompatible with the fundamental properties of matter, and in particular with the principle of the conservation of energy. Hence two kinds of determinism, two apparently different empirical proofs of universal necessity. We shall show that the second of these two forms is reducible to the first, and that all determinism, even physical determinism, involves a psychological hypothesis: we shall then prove that psychological determinism itself, and the refutations which are given of it, rest on an inaccurate conception of the multiplicity of conscious states, or rather of duration. Thus, in the light of the principles worked out in the foregoing chapter, we shall see a self emerge whose activity cannot be compared to that of any other force. [Sidenote: Physical determinism stated in the language of the molecular theory of matter.] Physical determinism, in its latest form, is closely bound up with mechanical or rather kinetic theories of matter. The universe is pictured as a heap of matter which the imagination resolves into molecules and atoms. These particles are supposed to carry out unceasingly movements of every kind, sometimes of vibration, sometimes of translation; and physical phenomena, chemical action, the qualities of matter which our senses perceive, heat, sound, electricity, perhaps even attraction, are thought to be reducible objectively to these elementary movements. The matter which goes to make up organized bodies being subject to the same laws, we find in the nervous system, for example, only molecules and atoms which are in motion and attract and repel one another. Now if all bodies, organized or unorganized, thus act and react on one another in their ultimate parts, it is obvious that the molecular state of the brain at a given moment will be modified by the shocks which the nervous system receives from the surrounding matter, so that the sensations, feelings and ideas which succeed one another in us can be defined as mechanical resultants, obtained by the compounding of shocks received from without with the previous movements of the atoms of the nervous substance. But the opposite phenomenon may occur; and the molecular movements which go on in the nervous system, if compounded with one another or with others, will often give as resultant a reaction of our organism on its environment: hence the reflex movements, hence also the so-called free and voluntary actions. As, moreover, the principle of the conservation of energy has been assumed to admit of no exception, there is not an atom, either in the nervous system or in the whole of the universe, whose position is not determined by the sum of the mechanical actions