Is the Brain a Digital Computer? Author(s): John R. Searle Reviewed work(s): Source: Proceedings and Addresses of the American Philosophical Association, Vol. 64, No. 3 (Nov., 1990), pp. 21-37 Published by: American Philosophical Association Stable URL: http://www.jstor.org/stable/3130074 . Accessed: 19/08/2012 03:34 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. American Philosophical Association is collaborating with JSTOR to digitize, preserve and extend access to Proceedings and Addresses of the American Philosophical Association. http://www.jstor.org IS THE BRAINA DIGITALCOMPUTER? JohnR. Searle Universityof California/Berkeley PresidentialAddress delivered before the Sixty-fourth Annual Pacific Division Meeting of the AmericanPhilosophical Association inLos Angeles, California, March 30, 1990. I. Introduction,Strong AI, WeakAI and Cognitivism. Thereare different ways to present a Presidential Address to the APA; the one I have chosenis simplyto reporton workthat I amdoing right now, on workin progress.I am goingto presentsome of my further explorations into the computational model of the mind.[1] Thebasic idea of the computer model of the mind is thatthe mind is theprogram and thebrain the hardware of a computationalsystem. A sloganone often sees is "themind is to thebrain as theprogram is to thehardware." [2] Letus beginour investigation ofthis claim by distinguishing three questions: 1. Is thebrain a digitalcomputer? 2. Is themind a computerprogram? 3. Can theoperations of thebrain be simulatedon a digitalcomputer? I willbe addressing1 and not 2 or 3. I think2 can be decisivelyanswered in the negative.Since programs are defined purely formally orsyntactically and since minds have an intrinsicmental content, it followsimmediately that the program by itselfcannot constitutethe mind. The formalsyntax of the program does not by itself guarantee the presenceof mental contents. I showed this a decadeago in the Chinese Room Argument (Searle,1980). A computer,me for example, could run the steps of the program for some mentalcapacity, such as understandingChinese, without understanding a word of Chinese. The argumentrests on thesimple logical truth that syntax is notthe same as, nor is itby itselfsufficient for, semantics. So theanswer to thesecond question is obviously"No". The answerto 3. seemsto me equallyobviously "Yes", at leaston a natural interpretation.That is, naturally interpreted, thequestion means: Is theresome description ofthe brain such that under that description you could do a computationalsimulation of theoperations of thebrain. But since according to Church'sthesis, anything that can be givena preciseenough characterization as a setof steps can be simulatedon a digital computer,it follows trivially that the question has an affirmativeanswer. The operations ofthe brain can be simulatedon a digitalcomputer in thesame sense in whichweather systems,the behavior of the New York stock market or thepattern of airline flights over LatinAmerica can. So ourquestion is not, "Is the mind a program?"The answerto that 21 22 APA PROCEEDINGS,VOL. 64,NO.3 is,"No". Noris it,"Can the brain be simulated?"The answerto thatis, "Yes". The questionis, "Is thebrain a digitalcomputer?" And for purposes of thisdiscussion I am takingthat question as equivalentto: "Are brain processes computational?" One mightthink that this question would lose muchof itsinterest if question 2 receivesa negative answer. That is, one might suppose that unless the mind is a program, thereis no interestto the question whether the brain is a computer.But that is not really thecase. Evenfor those who agree that programs by themselves are not constitutive of mentalphenomena, there is still an importantquestion: Granted that there is moreto the mindthan the syntactical operations ofthe digital computer; nonetheless, itmight be the case thatmental states are at leastcomputational states and mentalprocesses are computationalprocesses operating over the formal structure ofthese mental states. This, in fact,seems to methe position taken by a fairlylarge number of people. I amnot saying that the view is fullyclear, but the idea is something like this: At somelevel of description brain processes are syntactical; there are so tospeak, "sentences in thehead". Theseneed not be sentencesin Englishor Chinese,but perhaps in the "Languageof Thought" (Fodor, 1975). Now,like any sentences, they have a syntactical structureand a semanticsor meaning,and the problem of syntax can be separatedfrom theproblem of semantics. The problemof semantics is: Howdo thesesentences in the head get theirmeanings? But thatquestion can be discussedindependently of the question:How does the brain work in processing these sentences? A typicalanswer to that latterquestion is: The brainworks as a digitalcomputer performing computational operationsover the syntactical structure ofsentences in thehead. Justto keep the terminology straight, I call the view that all thereis tohaving a mind is havinga program,Strong AI, theview that brain processes (and mental processes) can be simulatedcomputationally, Weak AI, and the view that the brain is a digitalcomputer, Cognitivism. Thispaper is about Cognitivism, and I hadbetter say at thebeginning what motivates it. Ifyou read books about the brain (say Shepherd (1983) or Kuffler and Nicholls (1976)) youget a certainpicture of what is goingon in the brain. If you then turn to books about computation(say Boolos and Jeffrey, 1989) you get a pictureof the logical structure ofthe theoryof computation. Ifyou then turn to booksabout cognitive science, (say Pylyshyn, 1985) theytell you that what the brain books describe is reallythe same as whatthe computabilitybooks were describing. Philosophically speaking, this does not smell right to meand I havelearned, at leastat thebeginning ofan investigation,to follow my sense of smell. II. The PrimalStory I wantto beginthe discussion by trying to stateas stronglyas I canwhy Cognitivism hasseemed intuitively appealing. There is a storyabout the relation of human intelligence to computationthat goes back at leastto Turing's classic paper (1950), and I believeit is thefoundation ofthe Cognitivist view. I willcall it thePrimal Story: We beginwith two results in mathematicallogic, the Church-Turing thesis (sometimes calledChurch's thesis) and Turing'stheorem. For our purposes,the Church-Turing thesisstates that for any algorithm there is someTuring machine that can implement PRESIDENTIALADDRESSES 23 thatalgorithm. Turing's theorem says that there is a UniversalTuring Machine which can simulateany Turing Machine. Now if we putthese two together we havethe resultthat a UniversalTuring Machine can implementany algorithm whatever. Butnow, what made this result so exciting?What made it send shivers up anddown the spinesof a wholegeneration of youngworkers in artificialintelligence is the following thought:Suppose the brain is a UniversalTuring Machine. Well,are there any good reasons for supposing the brain might be a UniversalTuring Machine?Let us continuewith the Primal Story. It is clearthat at leastsome human mental abilities are algorithmic. For example, I canconsciously dolong division by going through the steps of an algorithmfor solving longdivision problems. It is furthermorea consequence of the Church-Turing thesis andTuring's theorem that anything a human can do algorithmicallycan be doneon a UniversalTuring Machine. I canimplement, forexample, the very same algorithm thatI use forlong division on a digitalcomputer. In sucha case,as describedby Turing(1950), both I, thehuman computer, and the mechanicalcomputer are implementingthesame algorithm; I am doing it consciously, the mechanical computer nonconsciously.Now it seems reasonable to suppose there might also be a wholelot ofmental processes going on in my brain nonconsciously which are also computational. Andif so, we could find out how the brain works by simulating these very processes ona digitalcomputer. Just as wegot a computersimulation ofthe processes for doing longdivision, sowe could get a computersimulation ofthe processes for understanding language,visual perception, categorization, etc. "Butwhat about semantics? After all, programs are purely syntactical." Here another set oflogico-mathematical results comes into play in thePrimal Story. The developmentofproof theory showed that within certain well known limits the semanticrelations between propositions can be entirelymirrored by the syntactic relationsbetween the sentences that express those propositions. Now suppose that mentalcontents in the head are expressed syntactically inthe head, then all we would needto accountfor mental processes would be computationalprocesses between the syntacticalelements in thehead. Ifwe get the proof theory right the semantics will takecare of itself; and that is whatcomputers do: theyimplement the proof theory. We thushave a welldefined research program. We tryto discover the programs being implementedin the brain by programming computers to implementthe same programs. We do thisin turnby getting the mechanical computer