_Third principle: The substance upon which the mind immediately acts--the psychode--is only susceptible of very simple modifications under the influence of the mind_, for, since the movements are to be somewhat varied, an extensive and complicated apparatus appears in the organism,--a whole system of muscles, vessels, nerves, etc., which are wanting in the inferior animals (among whom movements are very simple), and which would have been unnecessary had matter been directly susceptible of modifications equally varied under the influence of mind. When movements are intended to be very simple (as in the case of infusoria) the complicated apparatus is wanting and the life-spirit acts upon matter that is almost homogeneous. The following four hypotheses regarding the psychode may be formed: _a._ The psychode is a substance peculiar to the organism, and not capable of emerging from it. It acts only mediately upon everything outside of the visible organism. _b._ The psychode is a substance peculiar to the organism, capable of extending beyond the limits of the visible organism under certain special conditions. The modifications it receives necessarily act upon other inert bodies. The will acts upon the psychode, and thus mediately, upon the bodies that the sphere of this substance embraces. _c._ The psychode is a universal substance which is conditioned in its action on other inert bodies by the structure of living organisms, or by a certain state of inorganic bodies--a state determined by the influence of living organisms in certain special conditions. _d._ The psychode is a peculiar state of matter, a state habitually produced within the sphere of the organism, but which may also be produced beyond its limits under the influence of a certain state of the organism,--an influence comparable to that of magnets in the phenomena of diamagnetism. Thury proposes the adjective _ecteneic_ (from [Greek: ekteneia], extension) to describe that special state of the organism in which the mind can, in some measure, extend the habitual limits of its action, and he styles "ecteneic force" that which is developed in this state. The first hypothesis (he adds) would not be at all adapted to explain the phenomena with which we are concerned. But the three others give rise to three different explanations, in which (he assures us) the greater part of the phenomena investigated will be comprised. _Explanations based upon the Intervention of Spirits._--M. de Gasparin has shown the error of all these explanations: 1. By theological considerations. 2. By the very just remark that we should not resort to explanations which introduce spirits into the problem until other interpretations have been proved to be entirely insufficient. 3. Finally, by physical considerations.