PRESIDENTIAL ADDRESS—PRIESTLEY. 15 
the possibility of error in the metaphysical basis of the 
theory. It can be urged with some truth that this attempt 
resulted in valuable scientific progress. My point is that 
with the solution of the metaphysical problem left open 
there were two possible lines of advance, that actually 
followed at the time and that adopted thirty years later. 
The assumption of a particular solution closed one path; 
it is true that the path has been reopened, but the reopen- 
ing was possible only after philosophical criticism had been 
introduced. The moral is obvious. The tacit assumption 
of the answers to underlying metaphysical questions 
restricts the possible paths of scientific progress; to reach 
its full development Science must invoke the aid of 
Philosophy. ; 
A certain type of scientist would counter this state- 
ment with the assertion that he makes no metaphysical 
assumptions ; he is concerned not with metaphysical reality 
but with physical reality of which the criterion is the 
possibility of measurement. This reply suggests Johnson’s 
famous refutation of Berkeley’s philosophy. The actual 
observations made in a scientific measurement are observa- 
tions of coincidences; the completion of the process of 
measurement consists in the interpretation of these 
observations, and the interpretation involves metaphysics. 
It would be foolish and unnecessary to demand that the 
scientist should sclve the metaphysical problems, but he 
should at least reeognize when he assumes solutions, 
This somewhat cursory survey of the history of 
scientific thought reveals two main periods: the first 
characterized by crude metaphysical speculation and com- 
paratively barren; the second dominated by a return to 
experience, fruitful, but restricted in outlook by uncon- 
scious metaphysical assumptions. The pre-relativity work 
of Mach, Poincare, and others, and the general interest 
in the foundations of science that has accompanied the 
Theory of Relativity suggest that we are entering on a 
third period in whieh careful experiment will be combined 
with sound philosophical criticism. 
If this be so we can anticipate a period uo less fruitful 
than its predecessor, and characterized by a breadth of 
view which in the past has too often heen lacking, 
