ABSTRACT
The definition of validity used in standard ‘formal logic’ is this:
A philosophical argument is valid if it is not possible for the premises (assumptions) to be true and yet the conclusion to be false.
Put in other words, when a problem is expressed in the correct, logical way it can be known for certain that, if all the assumptions as to matters of fact are correct, then, because the problem has been addressed using the rules of logic, we can be certain that the conclusion is also true.