8 PROCEEDINGS OF THE ROYAL SOCIETY OF QUEENSLAND. 
adequate ordered description must provide the starting 
point of any further investigation. Some passages in 
Newton’s Principia have a distinctly modern ring; in 
particular the notes on the definitions show that many 
terms are intended to embody brief descriptions and carry 
no physical significance. For example, consider this 
passage :— 
“‘T use the terms ‘attraction’ and ‘impulse’ in the 
sense of ‘source of acceleration’ and ‘source of motion.’ 
I use ‘attraction,’ ‘impulse,’ ‘tendency towards a centre,’ 
indifferently and as mutually interchangeable; these are 
to be understood not as physical but as mathematical con- 
cepts. Hence, let the reader beware of thinking that when 
I speak of ‘the attraction of a centre’ or ‘central forces’ 
I mean to imply any particular mode of action, or physical 
cause, or to attribute real physical powers to the centres 
(which are mathematical points).’’ 
This quotation provides the key to the whole scheme 
of the Principia. The first two books are concerned with 
a mathematical description of a certain type of motion, 
and the third book discusses various problems im connec- 
tion with the motion of the planets, moon and comets, 
tidal phenomena, and the precession of the equinoxes. 
These are all explained in terms of gravitational forces 
corresponding to the ‘‘attractions’’ of the first two books. 
Thus the whole work is devoted to the investigation of how 
our system moves. ‘To extend the inquiry to the cause of 
this motion, it would first be necessary to pass from gravity 
as a convenient name to denote a source of acceleration 
towards a material particle, to gravity as an agency causing 
that acceleration by definitely explained means. 
Newton saw that this transition could not be made by 
means of further study of his phenomena, and, true to the 
new ideals of science, he refused to make it in any other 
way. The final Scholium to the Principia contains this 
passage :— 
‘““T have not been able to deduce the. reason of these 
properties of Gravity from the phenomena and I frame 
no Hypothesis. For whatever is not deducible from 
phenomena must be called hypothesis; and hypotheses 
whether metaphysical, or physical, or of occult qualities, 
or mechanical, have no place in Experimental Philosophy. 
