616 REPORT—1899. 
above a mere directory, with each individual event entered up separately with its 
address. But the similarities observed enable us to class large numbers of events 
together, to give general descriptions, and indeed to make, instead of a directory, 
a readable book of science, with laws as the headings of the chapters. 
These laws are, I believe, in all cases brief descriptions of observed similarities. 
By way of illustration let us take two or three examples. 
The law of gravitation states that to each portion of matter we can assign a 
constant—its mass—such that there is an acceleration towards it of other matter 
proportional to that mass divided by the square of its distance away. Or all 
bodies resemble 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 erroneous, of similarities which we believe 
we have observed. 
The old conception of laws as self-sufficing 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 result 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 occurred ; 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 predictions 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 commends analogous to legal enactments, and became mere descriptions. But 
during the passage from one position to the other, by a confusion of thought which 
