Logic & reason
Gil Harman has worked at this distinction and his views, usefully summarised below, provide at least some materials for reflection.
Difference 1 - reasoning, logic and belief revision
Reasoning is a procedure for revising one's beliefs, for changing one's
view, for modifying one's intentions and plans. This must not be
confused with argument for, or proof of, a conclusion, which is a
matter of proceeding from premises via a series of intermediate steps.
In fact Harman thinks that there is a category difference between
reasoning (reasoned change in view) and argument or proof. His point
is that a rule of argument is a principle of implication which tells us
that propositions of a certain sort imply other propositions of a certain
sort, but tells us nothing about belief-revision, so that 'rules of
argument are not by themselves rules for revising one's view' (Harman, 1986, p. 3).
The rules of logic are permissive rules licensing the deduction of
propositions. By applying such rules, we can generate more and more
logical consequences from an initial set. Now, in reasoning, Harman
points out, we not only add to our beliefs but sometimes subtract from
the beliefs held in store and this, he claims, illustrates one vital
difference between logical proof and reasoning. Another difference,
he suggests, is that, unlike logical principles, principles of belief
revision are defeasible - they don't hold in all instances. Consider, for
example the following principle of belief-revision: Tf one believes p
and also believes if p then q then one can infer q. Unlike modus
ponens, this principle does not always hold. Harman constructs a
counter instance:
Mary believes that if she looks in the cupboard, she will see a box of Cheerios. She
comes to believe that she is looking in the cupboard and that she does not see a box of Cheerios. At this point, Mary's beliefs are jointly inconsistent and therefore imply any
proposition whatsoever. This does not authorize Mary to infer any proposition what?
soever. Nor does Mary infer whatever she might wish to infer. Instead she abandons her
first belief, concluding that it is false after all. (Harman, 1986, p. 5)
In other words, if a logical consequence of some of the propositions
we believe is absurd or otherwise unacceptable, then the thing to do is
to revise or relinquish some of our beliefs, rather than to accept that
logical consequence.
Difference 2 - logic and the infinity of conclusions
Again, principles of logic permit the drawing of an infinite number
of conclusions from a set of premises. The premises p and q imply
p & q, (p & p) & q, (p v q) & q & q and so on. But it can hardly be a
principle of reasoning that we should accept (i.e., explicitly embrace) a
vast number of useless consequences of our beliefs, for that would be
to clutter the mind with useless trivialities (Harman, 1986, pp. 5-6,
12-13, 41-42). Harman makes much of this point, and advocates a
Principle of Clutter Avoidance which is 'a metaprinciple that con?
strains the actual principles of revision. The principles of revision must
be such that they discourage a person from cluttering up either
long-term memory or short-term processing capacities with trivialities'
(Harman, 1986, p. 15). For Harman, to have an explicit belief is to
have a representation - some sentence-like entity - in the brain
(Harman, 1973, 1977, 1986, pp. 12-14, 32) so clutter-avoidance is a
serious matter of preventing the brain bursting.
Difference 3 - logic, reasoning, and logical inconsistency
Finally, Harman objects to the idea that the relevance of logic to
reasoning is captured by the suggestion that, in reasoning, one seeks
to avoid logical inconsistency. Harman invites us to observe that we
sometimes discover that we hold inconsistent beliefs but, unable to
find a satisfactory way of revising them, we retain them 'while trying
not to exploit the inconsistency'. (Wittgenstein once took exactly this
line, but later changed his view.) Harman also points to situations
where one believes that not all one's beliefs could be true. He
comments that here one might well be justified in continuing to
believe that and each of one's other beliefs as well (Harman, 1986, pp.
15-16). Likewise, according to Harman, the proper, rational, response
to semantical paradoxes such as the Liar is to retain the Biconditional
Truth Schema '"p" is true if and only if p' as a kind of 'default
assumption', while recognizing that, when paradoxical sentences are
substituted for 'p' in the Schema, contradiction ensues. So the Schema
holds 'normally', but not without exception. Harman comments: 'This
does not seem to be a satisfactory solution from the point of view of
logic, since we take logic to require precise principles with precise
boundaries, not principles that hold merely 'normally' or 'other things
being equal'. But in ordinary life we accept many principles of this
vaguer sort (Harman, 1986, pp. 16-17).
Harman concedes that we do have the disposition to avoid embracing propositions we see to be inconsistent, but it is not just logical
inconsistency that we are so disposed to avoid. For example, we don't
accept both 'X is y's brother' and 'X is female' nor both 'Today is
Thursday' and 'Tomorrow is Saturday' (Harman, 1986, pp. 17-19). So
there is no special relevance of logic.
(Laurence Goldstein, 'Logic and Reasoning', Erkenntnis (1975-), Vol. 28, No. 3 (May, 1988), pp. 297-320: 297-9.
References
Harman, G.: 1973, Thought, Princeton University Press, Princeton, New Jersey.
Harman, G.: 1977, 'How to use Propositions', American Philosophical Quarterly 14,
173-76.
Harman, G.: 1979, '"If" and modus Ponens', Theory and Decision 11, 41-53.
Harman, G.: 1982, 'Logic, Reasoning and Logical Form' in T. W. Simon and R. Scholes
(eds.), Language, Mind and Brain, Erlbaum, New York.
Harman, G.: 1984, 'Logic and Reasoning', Synthese 60, 107-27.
Harman, G: 1986, Change in View, MIT Press, Cambridge, Massachusetts.
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