32 



NATURE 



[May 14, 1903 



Axiom il. ; there is the " grand " principle of the conserv- 

 ation of momentum, the algebraic sum total of it along 

 any direction in the universe being constant (and possibly 

 zero) by Newton's Axiom iii. ; as well as the " grand " 

 principle of the conservation of energy. 



Now I hold that it is invidious to apply laudatory epithets 

 of various degrees to these principles ; but it may not be 

 wrong to point out that in many respects the momentum 

 principle has a marked advantage over the energy principle, 

 the former being very often very easily, and without any 

 danger of error, applicable, while the latter (owing to the 

 elusiveness of energy) is full of danger to the unwary. 



Postulating now the existence of spirit, we find a difficulty 

 in defining this entity ; but no greater mystery attaches 

 to it than that involved in matter. The spiritualists imagine 

 that they gain something by calling matter hard and con- 

 temptuous names — " mere " matter, " gross " matter, 

 " mere gross " matter, &c. The names are harmless, and 

 do not assist ideas in any way. 



Postulating, then, the existence of a spiritual domain, the 

 crucial question arises : does Newton's Axiom iii. hold for 

 the interaction of the domains of matter and spirit? If it 

 does, there is no dynamical principle interfered with ; in the 

 dual domain there are conservations of force, of momentum 

 and of energy ; but in the physical universe, taken separ- 

 ately, neither force nor momentum would be conserved, 

 although energy might. On the other hand, if Newton's 

 Axiom iii. does not hold for the interaction of the two 

 domains, no principle of conservation could be enunciated 

 for either domain, or for the system of the two together. 



Sir Oliver Lodge is anxious to make out the existence 

 of a spiritual domain, and to allow it a certain influence on 

 the physical, which influence, however, " perturbs physical 

 and mechanical laws no whit." How does he effect this? 

 By assuming (to put the thing into mathematical language) 

 that the forces exerted on material things by the spiritual 

 are forces which do no work — such as are reactions of 

 smooth fixed surfaces, tensions of inextensible cords, &c. 

 These are sometimes called " deviating " forces. Sir Oliver 

 calls them " guiding and controlling " forces. But it 

 matters not what they are called, they fail in their object. 

 They allow, indeed, the physical universe to keep its sum 

 total of energy intact, but they infallibly alter its total 

 momentum and total force in every direction. 



When Sir Oliver says " guidance and control are not 

 forms of energy, and their superposition upon the scheme 

 of physics perturbs physical and mechanical laws no whit," 

 he says what is perfectly true of any conceivable forces^ — 

 whether merely " guiding " or not. However force may 

 be produced on a material particle, the effect on the particle 

 will certainly be in accordance with Newton's Axiom ii. ; 

 so that, in the sense in which Sir Oliver's statement is true, 

 there is no necessity to postulate that spiritual forces are 

 forces which act on matter but do no work. 



It is a physical and mechanical law that when any system 

 of material particles is subject to no forces but its own 

 internal forces, the centre of mass of the system is either 

 at rest or in uniform motion in a right line, and also that 

 Its sum total of energy, kinetic and dynamic, is constant. 

 But if Sir Oliver Lodge implies that both of these results 

 can remain unaltered if that material system is acted on 

 by spiritual forces, he is certainly wrong. His deviating, 

 or " guiding, " spiritual forces can leave the total energy 

 (kinetic and dynamic) of that material system unaltered, 

 hut they must inevitably interfere with the rest, or constant 

 motion, of the centre of mass. Many of his readers may 

 take this meaning out of his words; but I am sure Ihat he 

 cannot intend to be thus understood. 



It seems to me that Dr. Hobson in his letter on the 

 subject has done well to direct attention to the real status 

 of the " grand " principle of the conservation of energy. 

 George M. Minchin. 



Coopers Hill, Englefield Green, Surrey. 



Psychophysical Interaction. 

 As a psychologist I have read with deep interest Sir 

 Oliver Lodge's paper in your issue of April 23, and I write 

 to ask him to make clear some points which his paper leaves 

 obscure to my mind. Those of us who are not mathe- 

 maticians feel themselves to be very much at the mercy of 



NO. 1750, VOL. 6S'^ 



those who are, and we can only beg the physicists to re- 

 member our infirmity and to put their statements before us 

 in the clearest, simplest, and most carefully chosen language. 

 Sir Oliver Lodge, as Clerk Maxwell did before him, throws 

 out to psychologists the suggestion that mind may act upon 

 body by exerting guidance without doing work. Such 

 guidance, we are told, may be effected by the application of 

 force to moving masses in the nervous system in directions 

 perpendicular to the direction of the movements of those 

 masses. " Guidance is a passive exertion of force without 

 doing work ; as a quiescent rail can guide a train to its 

 destination, provided an active engine propels it." This 

 is the sentence that I find so indigestible. And my con- 

 fusion is but increased by Sir Oliver Lodge's further illustra- 

 tions. He distinguishes two kinds of force. " Force in 

 motion is a 'power,' it does work and transfers energy 

 from one body to another. But a force at rest — a mere 

 statical stress, like that exerted by a pillar or a watershed 

 — does no work, and alters no energy ; yet the one sustains 

 a roof which would otherwise fall, thereby screening a 

 portion of ground from vegetation ; while the other deflects 

 a rain-drop into the Danube or the Rhine." And, again, 

 we read that life can exert " the same kind of force which 

 can constrain a stone to revolve in a circle instead of in a 

 straight line ; a force like that of a groove or slot or 

 channel or ' guide.' " My first question is. Is it fair to 

 say that the pillar supporting the roof exerts a force in the 

 same sense as the rail which guides the train, the roof 

 which guides the rain-drop, or the hand which holds the 

 string? In the first case there is no motion, and therefore 

 no change of direction of motion, no alteration of energy ; 

 in the other cases there is motion and alteration of direc- 

 tion of motion. Secondly, is it fair to call the rail 

 quiescent? In guiding the train round a curve does not 

 the rail, and the mass to which it is made fast, suffer an 

 acceleration or a change of motion in the direction opposite 

 to that of the train? When I swing round a heavy ball 

 on a string, and feel it pulling my hand centrifugally, and 

 when by muscular effort I resist the pull, is that " a passive 

 exertion of force without doing work "? Or, if the string 

 is fastened to the end of an upright pole, is there not 

 movement of the mass to which the pole is fixed in the 

 direction opposite to the deflection of the movement of the 

 ball? Every kind of mechanical guidance that I can picture 

 to myself seems to imply action and reaction, change of 

 direction of one momentum seems to imply always an 

 opposite change of direction of an equivalent momentum. 

 This is, I suppose, the mechanical law of conservation of 

 momentum, of which Prof. James Ward tells us that it is 

 incompatible with the conception of guidance without work. 

 I ask Sir Oliver Lodge whether we are to understand that 

 he is prepared to throw this one mechanical law to the 

 wolves in order to preserve the rest of the creed of the 

 physicists unharmed by Prof. Ward's attack? Or are we 

 to understand that he repudiates the law of conservation 

 of momentum in toto ? In that case, I ask hjm to describe 

 for us clearly a single case of mechanical guidance in which 

 momentum is not conserved, or, since my use of the phrase 

 may be technically incorrect, I ask him to describe a case 

 of change of direction of motion of any mass produced 

 without expenditure of energy or opposite change of direc- 

 tion of motion of other mass or masses. 



I submit that Sir Oliver Lodge abstracts from the idea 

 of motion the attribute of direction in space, and that such 

 abstraction is illegitimate, save for certain purely theo- 

 retical purposes. AH motion has direction in space, which 

 would seem to be an essential element in all considerations 

 of energy values. Sir Oliver tells us that life and mind 

 cannot generate energy, though they can guide moving 

 masses by exerting forces perpendicular to the direction of 

 motion. But consider, then, the following case. Imagine 

 a universe consisting of two inert masses flying through 

 empty space in the same direction and at the same rate, 

 and a soul contemplating them. That universe would be 

 devoid of energy. Then suppose the soul to exert a force 

 upon one of the two masses, perpendicularly to its direction 

 of motion, so as to swing it round through a semicircle 

 until it rushes to meet the other mass. The soul, by 

 "guidance," has then created energy, and I take it that the 

 same considerations would hold true in our more complex 

 universe. 



