[FOM] Relevant logic and paraconsistent logic
A.P. Hazen
a.hazen at philosophy.unimelb.edu.au
Sat Mar 4 00:33:49 EST 2006
I've been slow in responding to Joseph Vidal-Rosset's question,
and I'm not sure I have much to add to the previous replies by
Lance, Heck, Mares and Avron: extra bibliography, perhaps.
(1) Relevant logics are systems in which contradictions don't
**imply** everything (A&~A)->B is not a thesis). Also, dually,in
which not everything implies a tautology (A->(Bv~B) -- or, to show
that this isn't about intuitionist scruples -- A->(B->B) is not a
theorem. In giving a "possible worlds" semantics, analogous to the
Kripke semantics of modal and intuitionistic logics, for such a
system we must allow there to be "worlds" in which contradictions are
true (and worlds in which tautologies fail). Thes peculiar "worlds,"
however, need not include the "actual" world: it is demonstrable that
the standard relevant logics are consistent (i.e. do not have
contradictions as theses), and are, in addition, "classically
complete," in the sense that they prove all the theses of classical
logic (the -> connective of these logics is non-classical, not
definable in terms of truth functions).
The formal work on the semantics of relevant logics does
not, of course, commit one to any particular stance on the
ontological status of possible worlds (and there have been violent
philosophical disagreements on such topics among the relevant
logicians!). Some of the early papers on the semantics of relevant
logics, by Richard Routley and Robert K. Meyer, avoided the term
"world," referring to the points in the model neutrally as "set-ups."
Another early contributor to the semantic study of these logics,
Alasdair Urquhart, suggested that the points be thought of as
THEORIES. (This was in line with some of the motivational remarks
made even earlier by Anderson and Belnap, who suggested relevant
logic was the appropriate framework for the history of philosophy:
one might want to say that a certain thinker was "committed" to P but
not to Q, even if that thinker was also committed to some
contradiction on another topic.
(BIBLIOGRAPHY: There are now a number of expository works on
the semantics of relevant logics. "Entailment, vol. II" by Anderson,
Belnap and Dunn has a 145-page chapter devoted to semantics. More
recent general introductions to relevant logics usually discuss the
Routley-Meyer ("Kripke-style") semantics: one possibility would be
the chapter (pp. 1-136) on "Relevance Logics" by Dunn and Restall in
volume 6 of the second edition of Gabbay & Guenthner,eds.,
"Handbook of Philosophical Logic" (2002). Its ancestor, Dunn's
chapter (pp. 117-124) in volume 3 of the first edition of said
hanbook 1986, is intrinsically very good, but has enough potentially
confusing typos to make me a little hesitant in recommending it for
private study. ... Mark Lance has written on interpreting relevant
logics in terms of "commitment".)
(2) The connection between "dialetheism" -- the view of
Graham Priest that there are true contradictions -- and the
Foundations of Mathematics is, I think, in need of further
clarification. Very early on in the study of relevant logics it was
suggested that one application would be the formulation or a set
theory based on the "naive" comprehension scheme. Such a system
would (unlike the pure logic) have to be negation inconsistent (it
would imply that the Russell set (exists, and) both is and is not a
member of itself). For theories based on classical -- or, for that
matter, intuitionistic -- logic, negation inconsistency is equivalent
to TRIVIALITY (a theory being trivial if every sentence of its
language is a theorem). Relevant logics, however, admit non-trivial
inconsistent theories. Someone accepting such a theory would be a
dialetheist. ... As other's have mentioned,
relevance logics are not the only possible bases for such theories.
There is a nice survey of (often 3-valued or 4-valued) systems in
Feferman's "Toward useful type-free systems, I" (JSL v. 75 (1984),
pp. 75-111). The logics of these systems typically differ from
relevant logics in NOT having an implication connective supporting
both modus ponens and a reasonable deduction theorem: Feferman
comments (p. 95) that "nothing like sustained ordinary reasoning
can be carried on in" them. (This is a bit out of context: the
comment was about the systems considerd early in his paper, and
perhaps shouldn't be generalized.)
(3) Relevant logic and dialetheism are NOT enough, however,
to allow for a non-trivial "naive" set theory. Consider Curry's
strengthening of Russell's Paradox, about the set (for any chosen
sentence C) of all things such that, IF they belong to themselves,
then C. ({x:x epsilon x-> C}) In a set theory based on naive
comprehension and any of the most familiar relevant logics, this
produces triviality. (BIBLIOGRAPHY: Curry's Paradox is discussed,
among other places, in the Feferman paper just cited. Its
application to "naive" set theories based on a variety of relevance
logics is nicely discussed by Robert K.Meyer, Richard Routley, and
J.M.Dunn, in (the English philosophical journal) "Analysis," vol. 39
(1979), pp. 124-128.)
(4) In the other direction, the project of using a
weaker-than-classical logic to allow a non-trivial "naive" set theory
does not require dialetheism: the proposition that the Russell set is
a member of itself might be allowed to lack a truth value (to be a
"gap") rather than being asserted to be a dialetheic "glut" (both
truth values). ...
(This was in effect oberved by Frederic Fitch in his
1952 book "Symbolic Logic": leaving the implication connective, and
so Curry's paradox, aside, Russell's paradox can be blocked by using
only a negation operator like Nelson's "constructible falsity") (=
"strong negation").) ...
Ross Brady was able to show that a
non-trivial set theory based on the naive comprehension scheme was
possible in a weak relevance logic (a weaker logic than those
considered by Meyer, Routley and Dunn in their "Analysis" essay).
One way of doing this produces a non-trivial but negation
inconsistent theory: this is favored by dialetheists like Priest.
Brady also discovered, however, that a negation CONSISTENT naive set
theory was possible in a slightly weaker logic (and showed this, I
think, by methods similar to those needed to establish the
non-triviality of his other system). My own (non-expert) view is
that this shows that, EVEN for those who want to save "naive" set
theory by working in a weak relevant logic, Dialetheism is an
optional extra.
(The BIBLIOGRAPHICAL bit that justifies my whole posting:
Ross Brady's monograph, "Universal Logic" has JUST been released by
CSLI press (distributed, I think, by University of Chicago Press), in
hardcover if you are a library and (we hope affordable) paperback for
human beings. It is THE place to go if you want to learn about naive
set theory in weak relevance logics!!!!!!!!!)
(5) Logical notes: The treatment of the implication
connective in the logics allowing non-trivial "naive" set theories
is like that of "lollypop" in Girard's Linear Logic, at least in
that the "Law of Contraction"
[P->(P->Q)]->[P->Q]
is not provable in them. (Otherwise they would trivialize, by
Curry's paradox.) Relevance logics typically DIFFER from Linear
Logic in that principles corresponding to all the laws of
DISTRIBUTIVE lattices hold for the extensional ("additive" in
Girard-speak, but relevance logicians were distinguishing
"extensional" from "intensional" connectives years before Girard
introduced his terminology) hold in them. These distribution laws
seem to make the logics more complicated in some ways: most
propositional relevant logics are undecidable, unlike propositional
Linear Logic without the exponentials. ... The logic of Brady's
negation-consistent "naive" set theory lacks the Law of Excluded
Middle.
---
Allen Hazen
Philosophy Department
University of Melbourne
(Colleague of Graham Priest, sometime student of Alan Anderson and
Nuel Belnap. All of whom at one time or another have criticized me
for being to classical or too Quinean in my philosophy of logic.)
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