The work may be considered, on the whole, as attached to the school of
Mill; to whose _System of Logic_, and to Bain's _Logic_, it is deeply
indebted. Amongst the works of living writers, the _Empirical Logic_ of
Dr. Venn and the _Formal Logic_ of Dr. Keynes have given me most
assistance. To some others acknowledgments have been made as occasion
arose.

For the further study of contemporary opinion, accessible in English,
one may turn to such works as Mr. Bradley's _Principles of Logic_, Dr.
Bosanquet's _Logic; or the Morphology of Knowledge_, Prof. Hobhouse's
_Theory of Knowledge_, Jevon's _Principles of Science_, and Sigwart's
_Logic_. Ueberweg's _Logic, and History of Logical Doctrine_ is
invaluable for the history of our subject. The attitude toward Logic of
the Pragmatists or Humanists may best be studied in Dr. Schiller's
_Formal Logic_, and in Mr. Alfred Sidgwick's _Process of Argument_ and
recent _Elementary Logic_. The second part of this last work, on the
"Risks of Reasoning," gives an admirably succinct account of their
position. I agree with the Humanists that, in all argument, the
important thing to attend to is the meaning, and that the most serious
difficulties of reasoning occur in dealing with the matter reasoned
about; but I find that a pure science of relation has a necessary place
in the system of knowledge, and that the formulæ known as laws of
contradiction, syllogism and causation are useful guides in the framing
and testing of arguments and experiments concerning matters of fact.
Incisive criticism of traditionary doctrines, with some remarkable
reconstructions, may be read in Dr. Mercier's _New Logic_.

In preparing successive editions of this book, I have profited by the
comments of my friends: Mr. Thomas Whittaker, Prof. Claude Thompson, Dr.