William James is widely regarded as America’s greatest philosopher.
He is also (with William Wundt) considered to be a father of
modern psychology.
Psychology was once an area of study within philosophy, and James was
for a number of years a philosophy professor. The distinction he made between
the two fields was this: Psychology was the “science of mental life”; that is, of
minds within a particular body, which exist in time and space, having thoughts
and feelings in relation to the physical world they are in. On the other hand,
explanations of thoughts as the product of some deeper force, such as the soul
or ego, were really the realm of metaphysics or philosophy. 
James considered this new subject a natural science that required analysis
of feelings, desires, cognitions, reasoning, and decisions according to their own
features and dynamics, in the same way that one would explain building a
house by looking at its stones and bricks. His choice to look at the phenomena
of psychology, rather than some theory behind them, advanced the subject
considerably and achieved his aim of putting it on a firmer scientific footing.
James was often depressed or in frail physical health, and The Principles
of Psychology took him all of 12 years to write. In his Preface he commented,
“it has grown to a length which no one can regret more than the writer him-
self. The man must indeed be sanguine who, in this crowded age, can hope to
have many readers for fourteen hundred continuous pages from his pen.” This
was the famous two-volume “long course,” the full version of the book. But
James also produced a condensed form, known as the “Jimmy” to college stu-
dents, who are grateful not to have to tackle the real thing.
Given its size, it would be presumptuous to “sum up” James’s masterpiece.
However, we look at a few ideas that hopefully give a flavor of its contents.