184 SECTIONAL ADDRESSES. 
ence it implies a single experience of difference, not a difference between two 
separate experiences; and that (iii) from the same standpoint we are 
quite unwarranted in adding together or subtracting from one another 
two intensities of sensation or two sensation differences as such. 
Elsewhere in his treatment of psychological data, as in his treatment 
of physical data, the mathematician has arrived at results that can directly 
be neither verified nor rejected by conscious experience. The establish- 
ment of ‘ general mental factors,’ involved in and influencing the per- 
formance of various mental tests and other processes, affords us another 
example. By mathematical operations on experimental data we can, it 
is claimed, deduce the existence of such general factors. But from the 
strictly psychological standpoint the nature of these factors cannot be 
interpreted ; for we are unable to appeal to direct experience to ascertain 
what these factors are. At best their significance in terms of actual 
experience can only be conjectured by abstraction and imagination; or 
it is expressible only in terms of general behaviour. At worst, as in the 
case of g (the so-called ‘ general intelligence’) we are quite ignorant of 
their psychical nature. We cannot hope for direct psychological evidence 
as to the precise mental nature of such mathematically deduced ‘ factors.’ 
It may be argued that the same ignorance holds in physics, say for 
electricity or gravity, the relations of which to other physical phenomena 
we can experimentally and mathematically determine, yet of the nature 
of which we are entirely ignorant through direct experience. But it may 
be retorted that electricity or gravity is an independent external non-mental 
activity, which is only related to conscious experience for its interpretation 
and conception ; whereas the results of psychological investigations must 
ultimately be expressible in terms of concrete conscious experience, not 
merely in terms of mathematical abstractions or of any physical activity 
which is fundamentally independent of such experience. 
With perhaps greater force it may be argued that introspection, by 
which alone conscious experience can be directly studied, is unreliable and 
not amenable to scientific methods—valid only for the particular individual 
who introspects, communicable to others only by outward behaviour, and 
fallible owing to illusion, rationalisation and other causes of error; and 
that just as physical experiment deals with such terms as electrons and 
quanta which are beyond the sphere of immediate experience, just as 
mathematical calculations yield for physics conceptions the realisation of 
which may be inconceivable by conscious experience, so modern experi- 
mental and mathematical psychology has a perfect right to express mental 
processes in terms which are foreign or even unknowable to conscious 
experience. 
PsycHoLoGy AND BEHAVIOUR. 
The escape from such difficulty on the physical side—so as to avoid 
dethronement of the literally divine claims which some mathematicians 
have made for the fundamental truth of their own science—is to regard the 
Universe as constituting a vast nexus of ultra-physical and mathematical 
necessities and probabilities, only some of which can become physical 
through the further operation of the human mind. We might perhaps 
adopt a corresponding attitude towards some of the subjectively 
