Are there logics with fairly natural semantics that don't have disjunction introduction? Is there a good way of thinking about what "flavor of system" you're in when you don't take disjunction introduction? Systems without double negation elimination seem to have a "constructive" feel to them. Is there a nice "family resemblance" of sorts between logical systems lacking disjunction introduction?
Here's a somewhat artificial example of a three-valued logic constructed not to admit disjunction introduction while still having fairly symmetrical truth tables and agreeing with classical logic when the intermediate truth value U
is ignored.
Here's a three-valued logic that doesn't admit disjunction introduction, and has the connectives K
(conjunction/koniunkcja), A
(disjunction/alternatywa), and C
(implication/implikacja). We pick both T
and U
as our designated truth-y truth values.
Kab b Aab b Cab b
F U T F U T F U T
(F) F F F (F) F F T (F) T T T
a (U) F U U a (U) F F T a (U) F T T
(T) F U T (T) T T T (T) F T T
So, we can see that disjunction introduction doesn't hold by demonstrating that CaAab
is a non-tautology. Let's assign both a
and b
the intermediate truth value U
.
CUAUU
CUF
F
U
is a designated truth value. If I takea
as a premise, then it can either have the truth valueT
orU
.a=T
would allow me to conclude thatAab
, buta=U
would not. In this section of the article on three-valued logic: en.wikipedia.org/wiki/… , LP and Kleene's logic have the same truth tables but different designated truth values.Aab
isT/U
butb
is false,a
has to beT
. Interestinglyb
beingU
is also enough to eliminate the disjunction.