Validity
It would be self-contradictory to affirm the premises and deny the conclusion. The corresponding conditional of a valid argument is a logical truth and the negation of its corresponding conditional is a contradiction.
The conclusion is a logical consequence of its premises.
An argument that is not valid is said to be ‘’invalid’’.
An example of a valid argument is given by the following well-known syllogism:
All men are mortal.
Socrates is a man.
Therefore, Socrates is mortal.
What makes this a valid argument is not that it has true premises and a true conclusion, but the logical necessity of the conclusion, given the two premises: the argument would be just as valid were the premises and conclusion false. (Although, whether or not an argument is valid does not depend on what anyone could actually imagine to be the case, this approach helps us evaluate some arguments.)
A standard view is that whether an argument is valid is a matter of the argument’s logical form.
Many techniques are employed by logicians to represent an argument’s logical form. A simple example, applied to the above two illustrations, is the following: Let the letters ‘P’, ‘Q’, and ‘S’ stand, respectively, for the set of men, the set of mortals, and Socrates.
Using these symbols, the first argument may be abbreviated as:
All P are Q.
S is a P.
Therefore, S is a Q.
Similarly, the second argument becomes:
All P are Q.
S is a Q.
Therefore, S is a P.
These abbreviations make plain the logical form of each respective argument. At this level, notice that we can talk about any arguments that may take on one or the other of the above two configurations, by replacing the letters P, Q and S by appropriate expressions.
Of particular interest is the fact that we may exploit an argument's form to help discover whether or not the argument from which it has been obtained is or is not valid. To do this, we define an “interpretation” of the argument as an assignment of sets of objects to the upper-case letters in the argument form, and the assignment of a single individual member of a set to the lower-case letters of the argument form.
Thus, letting P stand for the set of men, Q stand for the set of mortals, and S stand for Socrates is an interpretation of each of the above arguments. Using this terminology, we may give a formal analogue of the definition of deductive validity:
An argument is formally valid if its form is one such that for each interpretation under which the premises are all true also the conclusion is true.
As already seen, the interpretation given above does cause the second argument form to have true premises and false conclusion, hence demonstrating its invalidity.
Validity of statements
A statement can be called valid, i.e.
For example:
If no god is mortal, then no mortal is a god.
In logical form, this is:
If (No P is a Q), then (No Q is a P).
A given statement may be entailed by other statements, i.e. The corresponding conditional of a valid argument is a logical truth.
Validity and Soundness
One thing we should note is that the validity of deduction is not at all affected by the truth of the premise or the truth of the conclusion.
On this reading, formula is valid if all such interpretations make it true. An inference is valid if all interpretations that validate the premisses, validate the conclusion.
This is known as semantic validity.
Logical truths
Logical truths (including tautologies) are necessarily true. One theory is that a proposition such as “If p and q, then p” and the proposition “All husbands are married” are logical truths because they are true due to their inherent meanings and not because of any facts of the world.