CHAPTER VIII
SUMMARY AND CONCLUSIONS
To reduce my chief conclusions to a few brief statements
will be by no means easy, since the conclusions themselves
are necessarily so tentative. My primary object has been
to determine a little more exactly the nature of so-called
mentalc factors ' by examining the form of proof by which
those * factors' are established. I conclude that factors
as such are only statistical abstractions, not concrete entities.
To resolve a test-performance into ' g' and c s' no more
demonstrates the existence of a general and a specific
* ability 9 than describing a breeze as north-west implies
the combination of two currents from separate quarters
of the sky. We use factors in psychology as we use rectan-
gular co-ordinates in other sciences, not because we believe
that the phenomena investigated are necessarily dependent
upon a few, isolated, independent causal agencies, which
operate in the mind or brain, and which a sound method of
factor-analysis should successfully isolate, but merely because
such simplified descriptions enable us to organize our
facts into a more logical system and help us to state our
inductive arguments more cogently, and so endow our pre-
dictions with higher probabilities.
Mathematically, a factor is simply an average—usually a
weighted average—of certain measurements empirically
obtained. Logically, it is simply a principle of classification
—a principle by which both tests (or traits) and the persons
tested may be classified. Four kinds of factors may be
formally distinguished—(i) general, (ii) group or bipolar,
(iii) specific, and (iv) error factors, i.e. those possessed
by all the traits, by some of the traits, by one trait always,
or by one trait on the occasion of its measurement only.
This fourfold division broadly corresponds with the
traditional fourfold scheme of predicables, viz. gtnus,
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