11.1: What is Logic?

The field of logic is more prescriptive than descriptive. That is, it is not the study of how persons actually do reason, but rather the study of how they ought ideally to reason insofar as they are being "logical," that is, reasoning correctly. The field of logic also explores the errors to which thinking can be prone when it drifts away from the ideals of correct reasoning.

Logic in the narrowest sense of the term is not prescriptive and is only about logical consequence, that is, about what has to be true if something else is true. Our book uses a less narrow sense of what the term "logic" means.

Deductive logic explores deductively valid reasoning, the most secure kind of reasoning. The most elemental piece of reasoning is a simple argument that draws one conclusion from one premise or assumption. In any argument, simple or not simple, if the conclusion follows with certainty from the premise we call the argument deductively valid. A mathematical proof is an example of a complicated deductively valid argument.

Inductive reasoning, by contrast, is about less secure reasoning. Its conclusion follows from its premises with probability but not with certainty.

The field of logic explores the structural properties of reasoning. The field isn't interested in building bigger piles of good arguments, but in understanding their structural features. The structure is called "logical form." In this chapter we will be studying logical forms.