22. But this word 'art,' like most others, is ambiguous, and is
often used, not for skill displayed in practice, but for the knowledge
necessary thereto. This meaning is better conveyed by the term
'practical science.'

23. Science is either speculative or practical. In the first case we
study merely that we may know; in the latter that we may do.

Anatomy is a speculative science;
Surgery is a practical science.

In the first case we study the human frame in order that we may
understand its structure; in the second that we may assist its
needs. Whether logic is a speculative or a practical science depends
entirely upon the way in which it is treated. If we study the laws of
thought merely that we may know what they are, we are making it a
speculative science; if we study the same laws with a view to deducing
rules for the guidance of thought, we are making it a practical
science.

24. Hence logic may be declared to be both the science and the art
of thinking. It is the art of thinking in the same sense in which
grammar is the art of speaking. Grammar is not in itself the right
use of words, but a knowledge of it enables men to use words
correctly. In the same way a knowledge of logic enables men to think
correctly, or at least to avoid incorrect thoughts. As an art logic
may be called the navigation of the sea of thought.

25. The laws of thought are all reducible to the three following