# Deductively sound argument

Is there a way to prove that a deductively sound argument (where premises are true and arguments are valid) will always have a true conclusion?

• "Deductively valid" is ambiguous. It can mean an argument having a form that transforms true statements into true statements, or it can mean having a form that conforms to inference rules of a deductive system. In the latter case, you need the deductive system itself to also be sound, i.e. have rules that transform true statements into true statements. But, one way or the other, the proof is by definition: if you have a true statement (premise) and an argument that transforms true statements into true statements then the transformed statement (conclusion) is also true. Commented Sep 28, 2021 at 17:47

Valid argument means that:

it impossible for the premises to be true and the conclusion nevertheless to be false.

Sound means that the premises are true.

Therefore...

Is their a way to prove that a deductively sound argument(where premises are true and argument is valid) will always have a true conclusion?

By definition, the validity of a logical argument means that if the premises are true, then the conclusion is true.

By definition too, a sound argument is also logically valid.

Conversely, if you can exhibit a counterexample (that is to say, one case where the premises are true and the conclusion is false), then the argument is not valid, and therefore not sound.

Some arguments are so simple we are all very confident no one will ever find a counterexample. This applies for example to the modus ponens, the modus tollens and more generally to the small collection of logical truths identified by logicians since Aristotle.

However, our logical sense is extremely limited so that more complicated arguments are not so obvious and we are usually unable to decide whether they are valid or not. We can only decide by reasoning logically, much like Aristotle himself in Prior Analytics demonstrated that it can be done. Nothing has changed in this respect since then.

If all the premises are true and the argument is valid, then the conclusion must necessarily be true.

Therefore, all valid arguments with exclusively true premises have valid conclusions.