May 28, 1903] 



NATURE 



77 



LETTERS TO THE EDITOR. 



[The Editor does not Iwld himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the ivriters of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



Psychophysical Interaction. 



I MUST demur to the statement of my views which Sir O. 

 Lodge has given in his letter printed in Nature for May 14, 

 " that if dynamical laws are exact and irrefutable, the 

 universe must be a completely determined mechanical 

 system, with only one, and that a necessary, solution." 

 In the first place, I made no statement as to the universe 

 as a whole ; as I do not know the physical universe to be 

 finite in extent, I prefer to make statements only about 

 finite portions of the universe, and the interactions 

 of such finite portions. I certainly hold the view that the 

 laws of dynamics, which are a self-consistent system of 

 formal laws, are exact and irrefutable, but the question 

 whether the motions of all parts of a living organism are 

 in accordance with those laws is quite another matter, and 

 one on which I have expressed no opinion. What I did in 

 effect say, w^as that a material system upon which forces of 

 psychical origin and of incalculable magnitude acted, 

 traversed the laws of dynamics in the only sense in which 

 such a system of laws can be traversed, viz. that the 

 motions would not be in accordance with the laws, whether 

 the supposititious forces do mechanical work or not. 



Sir O. Lodge maintains that the psychical and the 

 physical can interact without upsetting any fundamental 

 dynamical law ; he objects to the principle of Least Action 

 as containing assumptions which beg the question at issue, 

 and pins his faith to Newton's laws. Now, although the 

 principle of Least Action contains nothing which is not 

 deducible from Newton's laws, provided the same form of 

 energy-function is taken in the two cases, I will, for the 

 sake of argument, accept the test that Sir O. Lodge lays 

 down. One of Newton's laws is that to every action there 

 is always an equal and opposite reaction, or every stress 

 has two aspects; now I suggest for Sir O. Lodge's con- 

 sideration the following questions :- — What are the reactions 

 corresponding to the forces of psychical origin which act 

 upon parts of a living organism? On what do such re- 

 actions act? It will clearly not suffice to say that the 

 reactions are something of a different character from the 

 actions, and are appropriate to exert an influence upon 

 the psychical ; Newton's reactions are mechanical forces 

 acting upon material systems. 



As an example of a mechanical system the motions of the 

 parts of which are determinate through the laws of dynamics 

 in conjunction with the law of gravitation, we may take 

 the solar system, supposing each member of it to be treated 

 as a whole. Let us suppose that there resided in the solar 

 system some agency of a non-material character which 

 was capable of applying to the planets forces of unknown 

 magnitudes along the normals to their orbits relative to 

 the centre of gravity of the system. The paths of the 

 planets could then no longer be calculated ; in fact, there 

 would be an end of gravitational astronomy ; both the 

 linear and angular momenta of the system, so far from 

 being conserved, would become absolutely indeterminate, 

 and yet Sir O. Lodge must in consistency maintain that 

 the laws of dynamics would not be traversed. Moreover, 

 although the sum of the potential energy and the kinetic 

 energy of the motions relative to the centre of gravity 

 would be unaltered, the energy of the motion of the whole 

 system through space would be altered to an unknown 

 extent. If the disturbing forces acted normally to the 

 paths relative to a point regarded as a fixed origin for the 

 sun and stars, the energy of the system would be con- 

 served, but in all other respects the same result as before 

 would ensue, namely, chaos. 



There are, I take it, in the main three views which may 

 be maintained as regards the relations of the psychical and 

 the physical in living organisms. 



(i) The view known as pure naturalism, that the 

 physical forms an independent system, and the psychical is 

 only a Begleiterscheinung influenced by, or perhaps deter- 

 mined by, the physical, but exerting no inflXience on the 

 NO. 1752. VOL. 68] 



physical. In this case the motions of the physical are 

 entirely determinate in accordance with mechanical laws. 



(2) The view that the psychical and the physical form 

 two systems linked together, with interaction between the- 

 two ; on this view neither system is complete in itself, and 

 the physical cannot be determined completely by any system 

 of purely mechanical laws. This view does not exclude 

 pure determinism as regards the whole complex, since it 

 may be held that the psychical has a dynamics of its own, 

 and that the interaction between the psychical and physical 

 is determinate in accordance with some scheme of laws. 



(3) Lastly, it may be held that the dualism of the physical 

 and psychical is entirely inadequate as an ultimate formula- 

 tion ; in fact, that both (i) and (2) are unworkable as 

 thorough-going hypotheses ; on this monistic view, both 

 the physical and the psychical must be regarded as manifest- 

 ations of something more fundamental than either. This- 

 view, as also (2), does not exclude the partial and tentative 

 application of mechanical laws, even to the case of living 

 organisms ; there may be a partial or practical indepen- 

 dence of the physical in certain classes or cases, but 

 such practical independence could never be presumed apart 

 from proof of its existence by means of actual observation, 

 and there must certainly be a point at which the practical 

 independence breaks down, and at which the dualism of 

 our ordinary mode of thinking becomes inadequate as a 

 representation of what happens. It is this last view of the 

 matter which I am inclined, personally, to regard as the 

 true one. E. W. Hobson. 



Christ's College, Cambridge, May 17. 



With the help of one of Clerk Maxwell's demons a very 

 simple illustration of change of motion in a dynamical 

 system, without any interference with the sums of energy 

 and momentum, can be constructed, which may perhaps, 

 be of service to Mr. McDougall. 



Let the demon provide himself with some inextensible, 

 perfectly flexible, mass-less string. (It is found abundantly 

 in text-books of Dynamics.) Let him observe two bodies 

 of the system, havjng, it may be, motions of rotation as well 

 as of translation ; and when he discovers a point on each 

 the relative velocity of which with respect to the other point 

 is either zero or at right angles to the straight line between 

 them, and which also are about to recede from each other, 

 let him, at the very instant when things are so, attach a 

 piece of his string to these two points exactly equal in 

 lepgth to the distance between them. The two bodies will 

 thus be suddenly yoked together without any shock whatever, 

 and consequently without any loss of energy. Their sub- 

 sequent motions of translation and rotation will be altered 

 by the action of the string ; but their total energy and their 

 total momentum will remain entirely unaltered. As soon as 

 the string slacks the demon must be careful to remove it, in 

 order to avoid the possible shock when it again tightens. 



If the string be perfectly elastic (so that no energy is 

 dissipated in internal work when the string stretches) in- 

 stead of inextensible, the demon may attach it to any two 

 points on the surfaces of the bodies without affecting the 

 momentum sum or the energy sum ; but so long as the 

 string is at all stretched, a portion of the energy of the 

 two bodies will be stored up in it. 



For example, let the two bodies be spheres moving with 

 the same uniform, rectilineal, velocity ; and suppose the 

 centre of figure of each to be its centre of inertia. Let each 

 be spinning about an axis through its centre, perpendicular 

 to the plane in which the centres are moving. Then the 

 demon may safely fasten his inextensible string to the two 

 points where the straight line joining the centres cuts the 

 surfaces. There will be no shock, and therefore no loss 

 of energy. There will be also no change in the total 

 momentum of the spheres, whether linear or angular, nor 

 any change in the uniform, rectilineal, motion of their 

 common centre of inertia ; nevertheless, when the demon 

 releases them, they may be moving in divergent instead of 

 parallel directions, and with diminished or increased 

 velocities of rotation. 



Demoniacal guidance of this kind conflicts neither with 

 the law of conservation of energy nor with that of the con- 

 servation of momentum, and so far would seem to 

 contradict Prof. Ward's criticism in his " Naturalism and 

 Agnosticism," vol. ii. p. 83. J. W. Sharph. 



Woodroffe, Bournerhouth. 



