Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uwm.edu!rutgers!njin!princeton!phoenix.princeton.edu!ghh From: ghh@clarity.Princeton.EDU (Gilbert Harman) Newsgroups: comp.ai.philosophy Subject: Re: Reasoning Paradigms Message-ID: Date: 9 Oct 90 01:05:16 GMT References: <9963@ccncsu.ColoState.EDU> Sender: news@idunno.Princeton.EDU Followup-To: comp.ai.philosophy Organization: Princeton University Cognitive Science Lab Lines: 58 In-reply-to: jmc@Gang-of-Four.usenet's message of 8 Oct 90 15:32:35 GMT In article jmc@Gang-of-Four.usenet (John McCarthy) writes: Minsky will doubtless tell you that logic isn't what he is enthusiastic about, whereas I am an enthusiast for logic. However, I wouldn't claim that logic "grounds" all reasoning, because I think grounding is an oversimplified notion. The human ability to do logic developed from and still uses processes that can be called reasoning but don't correspond to logic. These processes are inaccurate in unnecessary and inconvenient ways. These inaccurate human processes did form a desire to develop accurate reasoning processes, i.e. logic. As a branch of mathematics, logic is grounded in formal semantics as Tarski and others have described, i.e. it has been made independent of the thought processes that motivated its development. For full AI, mathematical logic needs supplements such as formalized nonmonotonic reasoning and probably formalized contexts, but these aren't reversions to ordinary soft thinking... It is important to distinguish the theory of implication (logic) from the theory of what you have reasons to conclude given the beliefs (and plans and goals) that you start with (the theory of reasoning). The theory of logic has nothing special to say about what reasoning a person ought to do (or is permitted to do). Logical theory does not make use of normative notions like "ought" or "permission". Nor does logical theory have a psychological subject matter. So logic is not about what to infer, if inference is a matter of arriving at certain beliefs. There are special deontic logics concerning the implications of propositions about what someone ought to do and there are special logics that concern the implications of propositions about beliefs. But these logics are themselves concerned with implications of certain propositions and not with what one should infer (i.e. believe) about these subjects. It is clear, of course, that people reason about implications. People reason when doing logic. But it is (or should be) equally clear that the principles of logic are not principles that people do or ought to follow in their reasoning. The principles of logic are simply not about what conclusions can be reached under certain conditions. Anyone tempted to suppose that there is a fairly direct connection between logic and some sort of reasoning might look at my discussion of this issue in CHANGE IN VIEW (MIT, 1986), chapters 1 and 2. I think I'm agreeing with Minsky and disagreeing with McCarthy. -- Gilbert Harman Princeton University Cognitive Science Laboratory 221 Nassau Street, Princeton, NJ 08542 ghh@clarity.princeton.edu HARMAN@PUCC.BITNET