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Transcript
One Problem with the Material Conditional
By: Erick Nelson
One problem with material implication, or material condition, is this: The denial of a
material conditional entails that the antecedent is true.
Example: “If God exists, then an All-powerful being exists.”
Using this notation:
This is equivalent to:
G -> A
~G v A
The denial is:
Or:
=:
~(G -> A)
~(~G v A)
G & ~A
So the denial of (G -> A) is (G & ~A)
But wait! This affirms that G (the antecedent) is True and A (the consequent) is false
But using our example sentence, this entails that “God exists” is True!
So, counter-intuitively, the person who denies that an All-powerful being exists if God exists
is logically committed to the assertion that God exists! A strange proof indeed.
The root problem seems to be the attempt to make conditionals fully truth-functional; that
is, to be able to assign T or F result for every combination of T/F values for the terms. But
is this “logical”? Is this even what we mean by if/then statements? Since, in a simple
conditional, we are only (explicitly) saying what happens if the first term is True, are we
always, or even ever, committed to a position regarding what happens if the first term is
False?
If we did this in computer programming, a lot of goofy things would happen. In typical
computer language logic, we say “if” something, then do something. Whenever the “if”
condition is false, the computer simply ignores the instruction and goes on to the next one.
I submit that even though in computer languages the conditional is actually an instruction
about what to do, rather than a description of what is the case, the same kind of logic
applies to conditionals in general.
So, what should prevail is at least a third value in addition to True and False, and that is
Unknown.
It just seems so wrong when Logic is illogical. Somebody ought to do something about it.
------------------------------Truth tables
G
T
T
F
F
A
T
F
T
F
~A
F
T
F
T
G->A
T
F
T
T
~(G->A)
F
T
F
F
G & ~A
F
T
F
F