PREFACE xi

Human Ability ; but the first Part has gained greatly
by his survey of the general field and his sympathetic
criticisms of my views. Among the numerous American
workers who have entered the field, to our great advan-
tage, the remarkably clear and systematic exposition of
Thurstone has proved most suggestive ; he wUl, I hope,
forgive me for taking his own tables as an occasional text
for my discussions.

To my recent colleagues, Dr. W. Stephenson and Dr.
A. J. Marshall, Research Assistants in the Department of
Psychology, this book owes an unusually heavy debt. They
have always been ready to read my notes, criticize my views,
and even check my calculations. Circumstances have lately
deprived me of their help : otherwise I should have held my
manuscript back, and profited still more fully from their
criticisms. To Stephenson, one of the most original and
vigorous of the many students who have worked both under
Spearman and myself, I am particularly grateful. Nothing
is more stimulating than the presence of an enthusiastic
collaborator, eager to explore a new field of work, yet attack-
ing it from an opposite angle instead of along identical lines,
On the two main issues I have just mentioned, his outspoken
criticisms, and above all the opportunities we have had for
personal discussion, have been invaluable at every point.
I believe that both the problems at stake and the alternative
solutions have been made at once clearer and more interest-
ing to the beginner, because I have thus been able to attack
them, not by a dogmatic pronouncement from one side, but
as part of a friendly and lively debate.

Throughout, however, my ultimate desire has been to
emphasize the agreement rather than the disagreement
between the various schools. When the differences are fairly
faced, the final outcome, I am convinced, is nearly always to
discover that the antagonistic doctrines are by no means so
incompatible as has been supposed : each has merely been
stressing a complementary aspect of the truth which the
opposite approach had missed or underrated. If, there-
fore, this book succeeds in establishing a unity of principle
underlying the mutual criticisms and the divergent views
of the various contributors, then one important step will