Logik wrote: ↑Fri Jan 18, 2019 10:32 am
Peter Holmes wrote: ↑Fri Jan 18, 2019 10:26 am
No, you misunderstand validity. The 'in any situation' condition is absolutely critical: in any situation
in which the premises are true, the conclusion is true. Sorry, but this is basic stuff.
It's so basic and I "misunderstand validity" so much that I can trivially recognise your errors.
In logic, an argument is valid if and only if it takes a form that makes it impossible for the premises to be true and the conclusion nevertheless to be false.
Corollary: In any situation
in which the premises are true, but the conclusion is false then the argument is INVALID
And while I "misunderstand the basics" let me point out your circular reasoning:
In any situation
how do you assert the truth-value of the premises, without falling for the "begging the question" logical fallacy?
I am not going to bother with the rest of your response until we can agree on logical discourse.
Let me clarify this. You wrote the following: 'To depend on deduction, your premise[s] must be universally true.'
By 'universally true', I take you to mean 'true in all situations'. And you go on to claim that we can't have or know universal truths - so that, in effect, deduction is impossible.
But you also offered the following example of (I assume) deductive validity:
'Premise: All humans are mortal
Premise: Logik is human.
True Conclusion: Logik is mortal.'
So my question is: do you or don't you accept the possibility of deductive validity? Please can you make up your mind?
If you do, then your claim that validity requires that premises are universally true is false. Consider the following.
P1 If splang is spleng, then spling is splong.
P2 Splang is spleng.
C Therefore, spling is splong.
This is deductively valid and complete gibberish. All that validity requires is that the conclusion follows from the premises. The truth-value of the premises is irrelevant - and that's why logic has nothing to say about their truth-value. And it's why an argument can be both valid and unsound.
You ask here: 'In any situation
how do you assert the truth-value of the premises, without falling for the "begging the question" logical fallacy?'
As I've explained, there's no need to assert the truth-value of the premises in order to construct a valid argument. That's why the in any situation in which the premises are true
condition is crucial. We're talking about validity, not soundness. (I assume you understand the difference.)