common to both (a Middle Term),
mediating between 'gods' and 'Alexander.' Mediate Inferences comprise
Syllogisms with their developments, and Inductions; and to discuss them
further at present would be to anticipate future chapters. We must now
deal with the principles or conditions on which Immediate Inferences are
valid: commonly called the "Laws of Thought."

Sec. 3. The Laws of Thought are conditions of the logical statement and
criticism of all sorts of evidence; but as to Immediate Inference, they
may be regarded as the only conditions it need satisfy. They are often
expressed thus: (1) The principle of Identity--'_Whatever is, is_'; (2)
The principle of Contradiction--'_It is impossible for the same thing to
be and not be_'; (3) The principle of Excluded Middle--'_Anything must
either be or not be_.' These principles are manifestly not 'laws' of
thought in the sense in which 'law' is used in Psychology; they do not
profess to describe the actual mental processes that take place in
judgment or reasoning, as the 'laws of association of ideas' account for
memory and recollection. They are not natural laws of thought; but, in
relation to thought, can only be regarded as laws when stated as
precepts, the observance of which (consciously or not) is necessary to
clear and consistent thinking: e.g., Never assume that the same thing
can both be and not be.

However, treating Logic as the science of thought only as embodied in
propositions, in respect of which evidence is to be adduced, or which
are to be used as evidence of other propositions, the above laws or
principles must be restated as the conditions of consistent argument in
such terms as to be directly applicable to propositions. It was shown in
the chapter on the connotation of terms, that terms are assumed by
Logicians to be capable of definite meaning, and of being used
univocally in the same context; if, or in so far as, this is not the
case, we cannot understand one another's reasons nor even pursue in
solitary meditation any coherent train of argument. We saw, too, that
the meanings of terms were related to one another: some being full
correlatives; others partially inclusive one of another, as species of
genus; others mutually incompatible, as contraries; or alternatively
predicable, as contradictories. We now assume that propositions are
capable of definite meaning according to the meaning of their component
terms and of the relation between them; that th