GarcherLancelot wrote:.. .
so what if there is no strict.. .principles of validity that is known?.. .
Logic has a set of rules to play by, like maths or chess - even better, like a computer programme.
If you apply those rules carefully to true statements, you can come up with new statements which are pretty well guaranteed to be true.
If you apply then to false statements or apply them wrongly, you will come up with new statements which are false. The old GIGO rule always holds true: Garbage In, Garbage Out.
Commonsense is, as you almost say, accumulated knowledge which has been internalised because it generally works well. But it is specific to each community in both time and space. To us now, it is common sense that germs cause disease; to our great-great grandparents is was a novel idea, and to New Guinea tribesmen is is crazy - everyone
knows malign spirits cause disease.
Both logic and commonsense are useful. Use them wisely!