SEPTEMBER 14, 1899 | 
NATURE 
47} 
The law of gravitation states that to each portion of matter 
we can assign a constant—its mass—such that there is an ac- 
celeration towards it of other matter proportional to that mass 
divided by the square of its distance away. Or all bodies re- 
semble each other in having this acceleration towards each other. 
Hooke’s law for the case of a stretched wire states that each 
successive equal small load produces an equal stretch, or states 
that the behaviour of the wire is similar for all equal small pulls. 
Joule’s law for the heat appearing when a current flows in a 
wire states that the rate of heat development is proportional to 
the square of the current multiplied by the resistance, or states 
that all the different cases resemble each other in having 
H +C?R¢ constant. 
And, generally, when a law is expressed by an equation, that 
equation is a statement that two different sets of measurements 
are made, represented by the terms on the two sides of the 
equation, and that all the different cases resemble each other in 
that the two sets have the constant relation expressed by the 
equation. Accurate prediction is based on the assumption that 
when we have made the measurements on the one side of the 
equation we can tell the result of the measurements implied on 
the other side. 
If this is a true account of the nature of physical laws, they 
have, we must confess, greatly fallen off in dignity. No long 
time ago they were quite commonly described as the Fixed 
Laws of Nature, and were supposed sufficient in themselves to 
govern the universe. Now we can only assign to them the 
humble rank of mere descriptions, often tentative, often erro- 
neous, of similarities which we believe we have observed. 
The old conception of laws as self-suffcing governors of 
nature was, no doubt, a survival of a much older conception of 
the scope of physical science, a mode of regarding physical 
phenomena which had itself passed away. 
I imagine that originally man looked on himself and the re- 
sult of his action in the motions and changes which he produced 
in matter, as the one type in terms of which he should seek to 
describe all motions and changes. Knowing that his purpose 
and will were followed by motions and changes in the matter 
about him, he thought of similar purpose and will behind all 
the motions and changes which he observed, however they oc- 
curred ; and he believed, too, that it was necessary to think 
thus in giving any consistent account of his observations. 
Taking this anthropomorphic—or, shall we say, psychical— 
view, the laws he formulated were not merely descriptions of 
similarities of behaviour, but they were also expressions of fixed 
purpose and the resulting constancy of action. They were 
commands given to matter which it must obey. 
The psychical method, the introduction of purpose and will, 
is still appropriate when we are concerned with living beings. 
Indeed, it is the only method which we attempt to follow when 
we are describing the motions of our fellow-creatures. No one 
seeks to describe the motions and actions of himself and of his 
fellow-men, and to classify them without any reference to the 
similarity of purpose when the actions are similar. But as the 
study of nature progressed, it was found to be quite futile to 
bring in the ideas of purpose and will when merely describing 
and classifying the motions and changes of non-living matter. 
Purpose and will could be entirely left out of sight, and yet the 
observed motions and changes could be described, and pre- 
dictions could be made as to future motions and changes. 
Limiting the aim of physical science to such description and 
prediction, it gradually became clear that the method was 
adequate for the purpose, and over the range of non-living 
matter, at least, the psychical yielded to the physical. Laws 
ceased to be commands analogous to legal enactments, and 
became mere descriptions. But during the passage from 
one position to the other, by a confusion of thought which 
may appear strange to us now that we have finished the journey, 
though no doubt it was inevitable, the purpose and will of 
which the laws had been the expression were put into the laws 
themselves ; they were personified and made to will and act. 
Even now these early stages in the history of thought can be 
traced by survivals in our language, survivals due to the ascrip- 
tion of moral qualities to matter. Thus gases are still some- 
times said to obey or to disobey Boyle’s Law as if it were an 
enactment for their guidance, and as if it set forth an ideal, the 
perfect gas, for their imitation. We still hear language which 
seems to imply that real gases are wanting in perfection, in that 
they fail to observe the exact letter of the law. I suppose on 
this view we should have to say that hydrogen is nearest to 
NO. 1559, VOL. 60] 
perfection ; but then we should have to regard it as righteous 
over-much, asort of Pharisee among gases which overshoots the 
mark in its endeavour to obey the law. Oxygen and nitrogen 
we may regard as good enough in the affairs of everyday life. 
But carbon dioxide and chlorine and the like are poor sinners 
which yield to temptation and liquefy whenever circumstances 
press at all hardly on them. 
There is a similar ascription of moral qualities when we judge 
bodies according to their fulfilment of the purpose for which we 
use them, when we describe them as good or bad radiators, good 
or bad insulators, as if it were a duty on their part to radiate 
well, or insulate well, and as if there were failures on the part 
of nature to come up to the proper standard. 
These are of course mere trivialities, but the reaction of 
language on thought is so subtle and far-reaching that, risking 
the accusation of pedantry, I would urge the abolition of all 
such picturesque terms. In our quantitative estimates let us be 
content with ‘‘high” or ‘‘low,” ‘‘ great’ or ‘‘small,” and let 
us remember that there is no such thing as a failure to obey @ 
physical law. A broken law is merely a false description. 
Concurrently with the change in our conception of physical 
law has come a change in our conception of physical explana- 
tion. We have not to go very far back to find such a statement 
as this—that we have explained anything when we know the 
cause of it, or when we have found out the reason why—a state- 
ment which is only appropriate on the psychical view. Without 
entering into any discussion of the meaning of cause, we can at 
least assert that that meaning will only have true content when 
it is concerned with purpose and will. On the purely physical 
or descriptive view, the idea of cause is quite out of place. In 
description we are solely concerned with the ‘‘how ” of things, 
and their ‘‘why” we purposely leave out of account. We 
explain an event, not when we know ‘‘why”’ it happened. but 
when we show ‘‘ how ” it is like something else happening else- 
where or otherwhen—when, in fact, we can include it as a case 
described by some law already set forth. In explanation, we 
do not account for the event, but we improve our account of it 
by likening it to what we already knew. 
For instance, Newton explained the falling of a stone when 
he showed that its acceleration towards the earth was similar to 
and could be expressed by the same law as the acceleration of 
the moon towards the earth. 
He explained the air disturbance we call ‘‘sound” when he 
showed that the motions and forces in the pressure waves were 
like motions and forces already studied. 
Franklin explained lightning when and so far as he showed 
that it was similar in its behaviour to other electric discharges. 
Here I do not fear any accusation of pedantry in joining those 
who urge that we should adapt our language to the modern view. 
It would be a very real gain, a great assistance to clear thinking, 
if we could entirely abolish the word ‘‘ cause,” in physical de- 
scription, cease to say ‘‘ why” things happen unless we wish to 
signify an antecedent purpose, and be content to own that our 
laws are but expressions of ‘Show ” they occur. 
The aim of explanation, then, is to reduce the number of 
laws as far as possible, by showing that laws, at first separated, 
may be merged in one ; to reduce the number of chapters in the 
book of science by showing that some are truly mere sub-sections 
of chapters already written. 
To take an old but never-worn-out metaphor, the physicist is 
examining the garment of nature, learning of how many, or 
rather of how few, different kinds of thread it is woven, finding 
how each separate thread enters into the pattern, and seeking 
from the pattern woven in the past to know the pattern yet to 
come. 
How many different kinds of thread does nature use ? 
So far, we have recognised some eight or nine, the number 
of different forms of energy which we are still obliged to count 
as distinct. But this distinction we cannot believe to be real. 
The relations between the different forms of energy, and the 
fixed rate of exchange when one form gives place to another, 
encourage us to suppose that if we could only sharpen our senses, 
or change our point of view, we could effect a still further reduc- 
tion. We stand in front of nature’s loom as we watch the 
weaving of the garment ; while we follow a particular thread in 
the pattern it suddenly disappears, and a thread of another colour 
takes its place. Is this a new thread, or is it merely the old 
thread turned round and presenting a new face tous? We can 
do little more than guess. We cannot get to the other side of 
the pattern, and our minutest watching will not tell us all the 
working of the loom. 
