More
than 23 hundred years ago, Aristotle noticed that in certain inferences it is
impossible for their premises to be true and their conclusions false. An
example is the inference from "All men are mortals" and "All mortals
fear death" to "All men fear death."
In
modern logic, such inferences are said to be deductively valid. Aristotle
discovered that the validity of an inference depends not on its subject matter,
but only on the form of the premises and conclusion. All inferences of the form
"All Fs are Gs, and All Gs are Hs, therefore All Fs are Hs" are
valid. He described a number of such forms, which are called
"syllogisms."
Until
the 19th century, the subject of logic pretty much consisted of Aristotle's
syllogisms. But syllogisms are only a small portion of all valid inferences,
and do not include many of the patterns of valid inference that are employed in
science and mathematics. In 1879 Gottlob Frege devised a much more general characterization
of valid inference that is sufficient for representing mathematical and scientific
reasoning. A descendant of Frege's system, called "First Order Logic with
Identity,“ is now generally thought to be capable of representing mathematical
theories and proofs, and is taught to all philosophy students.
Remember:
An inference (or argument) is valid when it is impossible for its premises to
be true and its conclusion false.
Additionally:
In the 20th century two great mathematical results were proved concerning first
order logic: it is complete, and it is undecidable. Kurt Godel demonstrated that
it is possible to program a computer to list all the valid inferences
(completeness), and Alonzo Church demonstrated that it is impossible to program
a computer to determine whether or not every inference is valid (undecidability).