ntion
of error, the organisation of reason against error. I have shown
that owing to the bent impressed upon it by the circumstances of its
origin, the errors chiefly safeguarded by the Aristotelian logic
were the errors of inconsistency. The other branch of Logic, commonly
called Induction, was really a separate evolution, having its origin
in a different practical need. The history of this I will trace
separately after we have seen our way through the Aristotelian
tradition and its accretions. The Experimental Methods are no less
manifestly the germ, the evolutionary centre or starting-point, of
the new Logic than the Syllogism is of the old, and the main errors
safeguarded are errors of fact and inference from fact.

At this stage it will be enough to indicate briefly the broad
relations between Deductive Logic and Inductive Logic.

Inductive Logic, as we now understand it--the Logic of Observation and
Explanation--was first formulated and articulated to a System of Logic
by J. S. Mill. It was he that added this wing to the old building.
But the need of it was clearly expressed as early as the thirteenth
century. Roger Bacon, the Franciscan friar (1214-1292), and not his
more illustrious namesake Francis, Lord Verulam, was the real founder
of Inductive Logic. It is remarkable that the same century saw
Syllogistic Logic advanced to its most complete development in the
system of Petrus Hispanus, a Portuguese scholar who under the title of
John XXI. filled the Papal Chair for eight months in 1276-7.

A casual remark of Roger Bacon's in the course of his advocacy of
Experimental Science in the _Opus Majus_ draws a clear line between
the two branches of Logic. "There are," he says, "two ways of knowing,
by Argument and by Experience. Argument concludes a question, but
it does not make us feel certain, unless the truth be also found in
experience."

On this basis the old Logic may be clearly distinguished from the new,
taking as the general aim of Logic the protection of the mind against
the errors to which it is liable in the acquisition of knowledge.

All knowledge, broadly speaking, comes either from Authority, _i.e._,
by argument from accepted premisses, or from Experience. If it comes
from Authority it comes through the medium of words: if it comes from
Experience it comes through the senses. In taking in knowledge through
words we are liable to certain errors; and in taking in knowledge
through the senses we ar