Sandra asked:

How may I be know if something that is logical for me is true?

and Carolina asked:

Is logic the same in all people, I mean; is it an established point of view? Does it develop through
time? Is it an authentic feeling?

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and Pablo asked:

How can we distinguish between good and bad logic?

Deductive logic lays out the principles of reasoning and provides laws for thought. It is true for you if
you are not irrational.

Take the Aristotelian syllogism:

All men are mortal

Socrates is a man

So Socrates is mortal

If you agree with the first two premises, it is inconsistent to disagree with the conclusion. This is a
paradigm of reasoning. The fact that men are mortal is known empirically because we know about
death. If we believed in afterlife, we could substitute "immortal" for "mortal" and the argument would
remain logically valid. This is because legitimate inferences are independent of subject matter. To
deny the argument's validity on the basis of subject matter you need to prove the falsity of a
premises, e.g. to say that it is not true that all men are immortal.

An argument which claimsto be logical, but where it is not inconsistent to disagree with the
conclusion if you agree with the premisses, would be an example of 'bad' logic.

One principle of reasoning is that you cannot believe a contradiction. We cannotbelieve that an
object is both round and square, so it is logically necessary that something cannot be both round and
square. This must be true for you.

Logical principles or the principles of logical argument don't change over time, but the content which
is the facts or premises we use and infer from do (e.g. beliefs about mortality).

The above principles are stated in formal logic which shows the structure underlying sentences.
Formal logic is a mechanical calculus which has enabled us to simulate thought processes in
computers. The most basic logic, propositional calculus, consists in the rules of inference governing
connectives which we use in language such as 'and', 'or', 'not', 'if...then...'. Predicate calculus is more
complex since it has introduced more symbols, such as one for existence, as well as variables. This
enables sentences such as those in the Aristotelian syllogism to be translated into formal language.

The extension of propositional calculus was a development rather than a change. However there can
be change. Susan Haack in Deviant Logicdistinguishes between systems that extend classical logic
and those which deviate from it. Classical logic holds that all our propositions are either true or false.
To take one example, there is a system of logic which introduces a third, indeterminate truth value
taken to be necessary to accommodate quantum theory.

Logic is not a feeling.

Rachel Browne