May 14, 1903] 



I^tATURE 



ZZ 



But this difficulty in conceivingf .that itiind or soul can 



iav a part in the world of matter by acting upon masses 



1 the brain exists only for those who persist in holding 



,.■ untenable hypothesis that all energy is kinetic energy, 



the motion of matter. This has proved, of course, an 



\cellent working hypothesis, but that it is true of all forms 



energy is nothing more than a pious hope. Yet it is 



u" definition of energy in these terms (tacitly or explicitly) 



at perpetuates the ancient difficulty of conceiving the rela- 



ins of mind and body, and it is the persistent adherence 



' this conception that, on the one hand, has landed so many 



inds in the absurdities of psychophysical parallelism, 



nd, on the other hand, drives so many others to refuse a 



' neral acceptance of the law of conservation of energy, and 



believe in an activity of the soul unconditioned by the 



(i.ist, a belief which destroys the rational basis of morals 



and renders a science of history and of society impossible. 



lo me it seems that this fundamental problem can only 



<■ properly stated when we cease to regard matter as the 



iiimate physical reality, when with Prof. W. Ostwald we 



AX, " Matter is no longer present for us as a primary con- 



1 cption ; it arises as a secondary phenomenon through the 



I nnstant coexistence of certain forms of energy. We shall 



iherefore have to frame the question in a new form — How 



are psychical phenomena related to the energy-concept? " 



and " that in the case of psychical processes we have to do 



with the rise and the transformation of a special kind of 



nf^rgy, which we, in order to be able to speak of it, will 



line provisionally psychical energy (geistige Energie)."' 



I laslemere, April 26. W. McDougall. 



I HAVE pleasure in answering Mr. McDougall 's questions 

 far as they are addressed to me. 



In the first place I have not presumed to say how psychic 

 vwiitrol actually is exercised; but, in contradistinction to 

 those who hold that control or guidance is impossible with- 

 out the generation or introduction of fresh energy, I have 

 pointed out that very simple and familiar mechanical 

 arrangements do constantly exert guidance without doing 

 any work ; for instance, a line of rails. 



Mr. McDougall thereupon asks me whether the line of 

 rails is really quiescent, whether it is not subjected to an 

 c pposite acceleration. I reply yes, but what of that? The 

 yield of rail is infinitesimal, but whatever its magnitude it 

 is such as to make the guidance less effective, not more ; it 

 is a passive yield to pressure, not an active exertion of 

 energetic work-performing forc6 in the direction of motion 

 or of change of motion. The recoil of a gun is of no assist- 

 ance in propelling a bullet. 



In so far as the rails yield to the train as it enters Euston 

 f by a curve, they guide it not to Euston as it was, but to a 

 slightly shifted destination. No matter, they guide it, and 

 they have had no energetic or propelling power whatever. 



He asks me further if I fully admit the principle of 

 universal equal opposite reaction, and the consequent con- 

 ' Tvation of momentum. 



Certainly I do; but I do not admit the (as I think) mis- 

 ken use Prof. James Ward makes of the principle in the 



ntence which he refers to. 



Mr. McDougall seems to overlook the fact that kinetic 

 rcrgy is independent of direction. Whether ' a thing be 

 moving north or south or east or west its energy depends 

 on mass and speed alone. To change the speed, work is 

 necessary ; no work is needed to alter the direction. 

 IVrhaps it may be a help to him, though it is not really 

 important in this connection, if I say thiit great momentum 

 does not necessarily imply great energy. The momentum 

 of a recoiling gun or earth is equal to that of the projectile, 

 but the energy of the projectile is enormous in comparison 

 with the energy of recoil. 



He asks me for an example of " a change of direction of 

 motion of any mass produced without expenditure of energy 

 ( r opposite change of direction of motion of other mass or 

 masses." But the two things are not the same. An 

 instance of change of direction of motion without expendi- 

 ture of energy is afforded by the instances we have already 

 taken, or by any perfectly elastic rebound — that of a comet 

 from the sun, for instance. Undoubtedly the sun thereby 

 acquires an equal opposite momentum, but what of that? 

 1 " Vorlesungen iiber Natiir('hilosophie " (Leipzig, 1902). 

 NO. I 750. VOL. 68] 



The modicum of" energy in this momentum is infinitesimal, 



for one thing, and for another it comeS frbni the comet, not 

 from the sun ; it renders the rebound less efficient, not 

 more ; it is no supply of energy from the central practically 

 stationary mass. The same thing is true of a ball whirling 

 on a string round a pole. When a boy holds the string in 

 an active hand, it is quite easy and usual to do a little 

 work by moving the hand a quadrant in advance of the 

 ball, and thus to maintain, or even increase, its energy ; 

 but the force so exerted by a hand is not purely radial, it 

 has a tangential component, and this part of it is effectively 

 propulsive. An energetic, not a passive, centre is needed 

 for that. 



Coming to another part of his letter ; his illustration of 

 a great display of available energy being brought about by 

 the reversal of motion of one of two similarly flying bodies, 

 suffers from the confusion of energy with available energy. 

 The flying of air molecules, for instance, is in every direc- 

 tion, sometimes so as to be able to collide, sometimes not, 

 but their energy is quite independent of these directional 

 circumstances. As Dr. Hobson truly says in your issue of 

 April 30, " the principle of energy, if applied to even the 

 simplest dynamical system which is possessed of more than 

 one degree of freedom, is, taken by itself, wholly in- 

 sufficient for the determination of the motion of such 

 system." That is one part of my contention, technically 

 stated. In so far as a question of absolute velocity seems 

 involved in the energy of a single isolated flying mass, I 

 might refer to a discussion of that aspect of the matter in 

 the Philosophical Magazine for October, 1898. 



In conclusion, I perceive that Mr. McDougall, like some 

 other philosophers, hopes to jump the admitted difficulties of 

 psychophysical interaction by ignoring " matter " altogether 

 and taking refuge in " energy " alone. I venture to predict 

 that those who attempt this will find that though they may 

 wander in dimness for a time, and may cultivate an un- 

 awareness of difficulties by failing to see them distinctly, 

 they will not derive any ultimate satisfaction from the 

 blindfolding ; nor do I think that they will be well advised 

 to transplant the definite physical term "energy," even 

 though prefixed by a special adjective such as geistige, in 

 order to denominate what is probably a perfectly distinct 

 psychical entity with laws of its own. Oliver Lodge. 



Those of your readers who have been interested by Sir 

 Oliver Lodge's article printed in Nature, April 23, on 

 the " Interaction Between the Mental and Material Aspects 

 of Things," may be glad to be referred to Thomas Solly's 

 essay on the Will, published in 1856. 



The suggestion of Solly is that every act of the will is 

 simply a guidance of mental activity, infinitesimal, indeed, 

 in its amount in each individual act, but such as to influence, 

 not the external world, but the character of the individual 

 exercising it, so that the sar.e external stimulus operates 

 after each successive act of the will on an individual whose 

 character has been changed by that act, whence same 

 stimulus is no longer necessarily the same motive. By 

 thus regarding each act of the will as a " self-determination 

 of the subject," the acts of choice or guidance are assumed 

 to take place in a region of activity about which we have 

 no physical information whatever, and the interactions of 

 material things are left absolutely untouched. 



The significance of the suggestion is made extremely 

 j clear in Solly's chapter on " Liberty, a Self-Determination 

 I of the Subject," and in subsequent chapters, by means of 

 1 very happily chosen geometrical illustrations. 

 I Mohuns, Tavistock, April 26. A. M. Worthington. 



Mendel's Principles of Heredity in Mice. 

 The issues raised in the case of these mice are as yet of 

 such a simple and familiar kind that the source of Prof. 

 Weldon's diffiiulty is not easy to surmise. When a gamete 

 G bearing albino and pink-eye meets a gamete G' bearing 

 coloured coat (in this case fawn) and pink-eye, a hetero- 

 zygote GG' was produced, with dark eyes and coloured coat. 

 Such hybrids, as the experiments proved, gave off equal 



