The aim of logic is to make explicit the rules by which inference may be drawn, rather than to study the actual reasoning processes that people use, which may or may not confirm to those rules. In the case of deductive logic, if we ask why we need to obey the rules, the most general form of answer is that if we do not contradict ourselves -or, strictly speaking, we stand ready to contradict ourselves. There is no equally simple answer in the case of inductive logic, which is in general a less robust subject, but the aim will be to find reasoning such that anyone falling to conform to it will have improbable beliefs.
A point to note is that the truth or falsity of the statements in an argument does not affect the logical relation of implication. Hence, conclusion may be validly drawn from false as well as true premises. The significance of this point is very clearly seen when it is realised that in science, and other areas of rational investigation, it is equally important to be able to test their consequences empirically. With respect to validity and truth, the following are the various relations that can exist between them: an argument may be valid regardless of the truth or falsity of the premises. If all the premises of a valid argument are true, the conclusion must be true. If all the premises are true, but the conclusion is false, then the argument is invalid; and an invalid argument may have premises and conclusion that are all true. For instance, premises: All lovers are stupid , and youths are lovers. Conclusion: Therefore all youths are stupid. The argument is valid, but it is not obvious that the statements are all true. Premises: All men are mortal, and all dogs are men. Conclusion: Therefore all dogs are mortal. The argument is valid, the conclusion is true, but at least one of the premises is false.
However, when an argument is of the best quality it is not only valid but also has all its constituent statements being true. When this obtains, we say that the argument is sound. Indeed, it is possible to have a deductively valid argument which is made up of false premises. This, then, is one of the limitations of formal logic.
Apart from what may be called logical or a prior truth (also called truth of reason), all other information upon which reasoning is to be performed must come directly or indirectly from experience, mostly sense experience, observation, and experiment. Logic can lead to the discovery of truths and the solution of problems only if the available information is good, especially in terms of being contextually correct, sufficient, relevant and current. When the information is deficient, logic cannot be either a remedy or a substitute.
We need therefore to pay as much attention to our method of collecting, preserving and transmitting information, as we should to the logical processing of information in any field or endeavor whatsoever.