WHAT IS LOGIC?
Jaakko Hintikka, Gabriel Sandunevertheless reasonable to identify logic as the study of inferences and inferential
relations. The obvious practical use of logic is in any case to help us to reason
well, to draw good inferences. And the typical form the theory of any part of logic
seems to be a set of rules of inference.
This answer already introduces some structure into a discussion of the nature of
logic, for in an inference we can distinguish the input called a premise or premises
from the output known as the conclusion. The transition from a premise or a
number of premises to the conclusion is governed by a rule of inference. If the
inference is in accordance with the appropriate rule, it is called valid. Rules of
inference are often thought of as the alpha and omega of logic. Conceiving of logic
as the study of inference is nevertheless only the first approximation to the title
question, in that it prompts more questions than it answers. It is not clear what
counts as an inference or what a theory of such inferences might look like. What
are the rules of inference based on? Where do we find them? The ultimate end
one tries to reach through a series of inferences is usually supposed to be a true
proposition. Frege [1970, 126] wrote that “the word ‘true’ characterizes logic.” But
how does this desideratum determine the rules of inference? A few distinctions
will illustrate the embarrassment of riches covered by the term “logic” and at the
same time sharpen the issues