soundness: how can one decide\know if premise is true or false

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An argument is sound if and only if

1. The argument is valid.
2. All of its premises are true.

For instance,

1. All men are mortal.
2. Socrates is a man.
3. Therefore, Socrates is mortal.

The argument is valid (because the conclusion is true based on the premises, that is, that the conclusion follows the premises) and since the premises are in fact true, the argument is sound.

There is nothing wrong in my perspective, but what if there is a religion/person that does not believe in mortality. For such religion/person all men are mortal is FALSE therefore argument is not sound.

So, how can one decide\know if premise is true or false?

That opens another question, why is there soundness property at all?

I am sorry if this kind of question was covered in another thread.

2Some of us are interested in trying to found/discover/ establish truth: in philosophy, science, ... Thus, assuming that we have found some truth (also provisionally) we want to deduce other truth from them by valid argument. Logic is basically interested into finding valid way of reasoning. – Mauro ALLEGRANZA – 2014-11-11T16:58:52.847

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In general, you do not know whether the premise is true or not. Still, we would like to say that something is wrong with an argument, if its premises are wrong. Note that

All men are immortal.
Socrates is a man.
Therefore, Socrates is immortal.


is a valid argument. But presumably, it is not sound.

When calling an argument "sound", you defer the questions of knowledge and certainty. But usually, an argument should not only have premises that happen to be true, but premises that we find plausible (in which case, you might call it an persuasive argument).

You can have arguments that are valid, sound, persuasive; but usually not absolutley certain.