contingent. By 'contingent' is meant what holds true in some cases,
but not in others. For instance, in the particular case of equilateral
triangles it is true to say, not only that 'all equilateral triangles
are equiangular,' but also that 'all equiangular triangles are
equilateral.' But the evidence for these two propositions is
independent. The one is not a formal consequence of the other. If it
were, we should be able to apply the same inference to all matter, and
assert generally that if all A is B, all B is A, which it is notorious
that we cannot do.

18. It remains now for the full elucidation of our definition to
determine what is meant by 'science.'

19. The question has often been discussed whether logic is a science
or an art. The answer to it must depend upon the meaning we assign to
these terms.

20. Broadly speaking, there is the same difference between Science
and Art as there is between knowing and doing.

Science is systematized knowledge;
Art is systematized action.
Science is acquired by study;
Art is acquired by practice.

21. Now logic is manifestly a branch of knowledge, and does not
necessarily confer any practical skill. It is only the right use of
its rules in thinking which can make men think better. It is
therefore, in the broad sense of the terms, wholly a science and not
at all an art.