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Chapter 1Lo gic and t h e Stu dy of Human Rea soni ng

1.1 Why we re p s y c h o lo g i s t s i n t e r e s t e d i n d e d u c t i v e l o g i c ?The world, i n s h o r t , was p r o v id in g n o t sensation b u tfodder fo r our hypotheses . -- Jerome Bruner*For many, t he deg ree t o which human le a r n i n g mechanisms can

be counted on t o produce va l i d knowledge i s t h e measure of man'sr a t i o n a l i t y . Bu t what c h a r a c t e r i s t i c s must a l e a r n i n g p r o c e s shave t o ensu re th a t t he knowledge acqu i red i s va l i d? Because th eg e n e r a l i z a t i o n s o f s c i e n c e a f f o r d t h e c l o s e s t a pp ro ac h t o what wein tu i t i v e l y t h in k of a s v a l i d know ledge, p s y c h o lo g i s t s ha vew atch ed t h e p h il o so p hy of s c i e n c e c lo s e ly t o l e a r n whichc h a r a c t e r i s t i c s of t h e s c i e n t i f i c l e a rn i n g p r oc e ss a r ee p is t em o lo g ic a ll y c r i e r i a l . If everyday learning can be shownt o s h ar e t h es e c r i t e r i a 1 c ha r a c t e r is t i c s w i th i t s more re f ineds i s t e r , t h e n human r a t i o n a l i t y i s spared .

Ever s in ce Hume, indu ct io n has ca r r i ed a heavy l oad i npsychology whi l e t a k i ng a sound ep is temo log i ca l bea t ing . Inpsychology, it h a s been t h e l e a r n in g t h e o r y of c h o i c e s i n c e t h eB r i t i s h E m p i r i c i s t s ar gu ed t h a t t h e e x p er i en c e of s p a t i a l l y a ndtempora l ly con t iguous even ts i s what all ow s us t o jump from th ep a r t i c u l a r t o t h e g e n e r a l , from s e ns a t i o n s t o o b j e c t s , fromob je c t s t o concep ts . Pav lov ian re f lexo logy , Watson ian andSkinner ian behav ior i sm, even the sensory-motor pa r t s of P iag e t i ans t ru c t u r a l i s m have been mere essays on th e i ndu c t i ve psycho logy

* Bruner, 1984, p. 95, emphasis hi s.6


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of th e B r i t i s h Em pi ri ci st s. Yet when Hume, a proponent ofi n d u c t i v e i n e r e n ce a s a p s y c h o l og i c a l l e a r n i n g t h e o r y , donned h i sph i lo soph er ' s ha t , he showed t h a t i nduc t ion cou ld never j u s t i f y aun iv er sa l sta tem en t. Thus, Hume showed t h a t th e pr oce ss by whichpeople were presumed t o l ea rn about t he world could not ensuret h a t t h e g e n e r a l i z a t i o n s it produced would be valid.

Only recent ly , wi th t h e pub l ic a t i on i n 1959 of Kar l Popper I sThe Loaic pf S c i e n t l f l c D is co ve rv. . , h a s t h e p h i l o s o p h i c a l b e a t i n gof psychology vs fa v or i t e le ar ni ng theory subs ided. Popper arguedt h a t a l though a univ ers a l s ta te ment of sc i enc e can never be provedt r u e , i t d e d u c t i v e l y i m p li e s p a r t i c u l a r a s s e r t i o n s a bo u t t h e wo rl d-- hypotheses -- and pa r t i c u l a r a s s e r t ion s can be p roved fa l s e .No number of o bser ved whi te swans can prove t h a t "Al l swans a r ew hi t en i s t rue, but just one black swan can prove i t f a l s e .G ener a l i z a t ion s canno t be confi rm ed , bu t they can be f a l s i f i e d ,s o i n d u c t i o n s t e s t e d v i a d e d uc t i on s a r e on f i r m e r e p i s t e m o l o g i c a lground th an knowledge produced through i nd uc ti on alon e.

Th i s view had consequences fo r psycho log i s t s i n t e r es t e d i neveryday lea rni ng, I f one assumes t h a t the evolu t ion ary purposeof human learning i s t o p ro du ce v a l i d g e n e r a l i z a t i o n s a b ou t t h eworld, the n su re ly everyday le ar ni ng must be some form ofPopperi an hypo thes i s t e s t in g . On t h i s v iew, induc t ion s t i l lp lay s an im por tant r o l e i n the c r ea t ion of knowledge -- i t i s asource of t e s t a b l e hypo theses . But the burden of va l i da t i ngknowledge now fa l l s on dedu ct iv e log ic . Indu ct iv e processesmight suggest "If P t hen Q" -- and "If R then Q" , " I f S t hen Q",and so on -- b u t i t i s our use of deduc t ive lo g i c th a t causes ust o r e j e c t t h e h y p o t h e s i s i f Q t u r n s o u t t o be f a l s e when P ( o r R


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o r S ) i s t r u e . The r e p e a te d a p p l i c a t i o n of d e d u c t i v e l o g i c i nt e s t i n g t h e many i n d u c t i v e l y p ro du ce d h yp o th e s es e x p l a i n i n g Ql e t s u s h one down t h e p o s s i b i l i t i e s a nd z e r o i n on t h e t r u t h . Ont h i s vi ew , human l e a r n i n g p r o c e s s e s w i l l produce va l id knowledget o t h e e x t e n t t h a t t he y us e de d uc ti ve l o g i c t o f a l s i f yhypotheses .

T h i s s h i f t s t h e o r e t i c a l p r i o r i t i e s i n psychology from t h es t u d y of i n d u c t i v e p r o c e ss e s t o t h e s t ud y of d e d u c ti v e l o g i c .E i t h e r everyday human l e ar n i ng mechanisms usu a l l y p roduce in va l i dk now led ge, o r t h e y i n c l u d e a l g o r i t h m s t h a t f r e q u e n t l y a nds p o nt a ne o u sl y a p p ly d e d u c ti v e l o g i c i n t e s t i n g h y p ot he s es .

A l e g i o n of c o g n i t i v e p s y c h o l o g i s t s , fr om P i a g e t ( e g .,I n h e l d e r & P ia g e t , 1958, p. 254-255) t o Bruner (Br une r, Goodnow &Aus t in , 1956) t o Wason & Johnson-Laird (1972) t o Fodor (1975)h av e ad o pt ed t h i s Po pp er ia n v iew o f h y p o t he s i s t e s t i n g a s t h e i rmodel of human le ar ni ng . To pa ra ph ra se Fodor , who was speakingf o r t h e f i e l d , h y po th es is t e s t i n g i s t h e on l y t heo ry we 've g o t(Fodor, 1975, Ch. 1 ).

D e d uc t iv e l o g i c h a s a n o t h e r p r o p e r t y t h a t was t e m p t in g t oc o g n i t i v e p s y c h o lo g i s ts : it i s conten t - independent . The r u l e s o fi n f e r e n c e of t h e p r o p o s i t i o n a l c a l c u l u s * g e n e r a t e on ly t r u econc l u s i ons f rom t r u e p rem i se s , r eg a rd l e s s o f what t hep r o p o s i t i o n a l c o n t e n t of t h o s e pr e m is e s i s . T h e p r o p o s i t i o n a lc a l c u l u s i s t h e p e r f e c t i n e rence eng i ne fo r a dom ai n ge ne ra lin fo rma t ion proce ss in g syst em: no mat t e r what hypothesesi n d u c t i v e p r o c e s s es f e e d i t , i t w i l l o u t p u t on l y v a l i dconc l u s i ons . The i dea t h a t t h e human mind has a l go r i t hm s t h a t

* t h e p h i l o s p h e r ' s name f o r f o r m al p r o p o s i t i o n a l l o g i c ( Q u in e , 1 9 5 0 ) .8


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i n s t a n t i a t e t h e r u l e s o f i n f er e n c e of t h e p r o p o s i t io n a l c a l c u l u sf i t w e l l w it h c o g n i t i v e p sy ch olo gy ' a meta-theory .

Consequent ly , many ps ych olo g i s t s have spen t a g r ea t de a l o ft ime and e f f o r t i n sear ch of a "deduc t iv e component" (Wason &Johnson-Lai rd, 1972), o r , i n more c u r r e n t p a r la n c e , a l o g i c"module ." The th e o r e t i c a l burden they have p lac ed on t h i sproposed menta l a lgor i thm i s s t a g g e r i n g : I t i s supposed t o ben e c e ss a r y f o r b u i l d i n g v i r t u a l l y a l l t h e v a s t a nd complex

I

knowledge s t r u c tu r e s t h a t power human though t and behavior , f romt h e most u b i q u it o u s of s o c i a l i n t e r a c t i o n s t o t h e most e s o t e r i cf e a t of modern technology .1.2 What would a l o g i c modu le b e l i k e ?

The "d oc t r in e of menta l l o g ic n (Johnson-Lai rd , 1982) i s t h ev iew t h a t t h e human mind i nc l ude s i nna t e a l go r i t hm s i n s t a n t i a t i n gt h e r u l e s of i n f e r e n c e of t h e p r o p o s i t i o n a l c a l c u l u s -- a l o g i cmodule. What p r op er t i es can a lo g ic module be e x pe c te d t o h av e?

Chomsky ( 1975 ) , Marr & Nishihara (1978) , and Fodor ( 198 3)have t aken t h e b i o l og ic a l v iew (b es t summarized by Wi ll i ams ,1 9 6 6 ) t h a t i f a f u n c t i o n i s e v o l u t i o n a r i l y i m p o rt an t , n a t u r a ls e l e c t i o n w i l l produce a species-wid e psy ch olo gic al mechanismw i t h c e r t a i n p r o p e r t i e s . Namely t

1. I t w i l l be s p e c i a l l y d e si g ne d t o s o l v e t h e e v o l u t i o n ar yproblem q u i c k l y , r e l i a b l y , and e f f i c i e n t l y . Consequent ly , itw i l l i n s t a n t i a t e m en ta l a r c h i t e c t u r e and r u l e s o f i n f er e n cet h a t w i l l d e f i n e t h e e v o l u t i o n a r i l y s a l i e n t d im en si on s of t h eproblem, and gu ide t he organ i sm toward an ad apt ive lya p p r o p r i a t e s o l u t i o n .

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2 . I t w i l l be domain sp ec if ic . Only by l i mi t i ng i t s scope ofa p p l i c a t i o n c a n it b e s p e c i a l l y d e s ig n e d t o s o l v e t h e pr ob le mq u i c k ly and e f f i c i e n t l y . I would add t h a t i t must have de si gnf e a t u r e s t h a t make i t s e n s i t i v e t o c ue s t h a t i n d i c a t e when t h eorganism has encou ntere d t h e domain fo r which th e mechanismwas des igned. An a lgor i thm th a t a l lows you t o dec ide betweenf i g h t o r f l i g h t i n t h e pr es en ce of a p re da to r i s u s e l e s su n l e s s it h a s f e a t u r e s t h a t l e t you d e te r mi n e what c o u nt s a s apre da t or and when you a r e i n th e presence of one .

3. I t w i l l d ev e lo p w i t ho u t e x p l i c i t t e a c h i ng o r t r a i n i n g .Exposure t o t he doma in may be neces sary t o a c t i v a t e t hemechanism or t o allo w it t o f i l l i n p ar am et er va lu e s. But t h er u l e s t h a t o r g a n iz e and pr o ce s s t h e s t i m u l i a r e i n n a t e l ys p e c i f i e d .

4 . The infe rences w i l l be made au tomat ica l ly , wi thout thea p p l i c a t i o n of " co ns ci ou s e f f o r t n o r d e l i b e r a t i o n . T h i s i s aconsequence of i t s h av in g t o be f a s t a nd r e l i a b l e ( t o r em ai nr e l i a b l e , t h e r u l e s must b e p r o t e c t e d from t h e e f f e c t s ofd e l i b e r a t i o n -- they cannot be " i so t r op ic n (Fodor 1983) .

Fo llowing t h i s vi ew , a l o g i c module neces sary fo r gene ra t ing vas tknowledge s t r u c t u r e s o ug ht t o h av e s e v e r a l p r o p e r t i e s :

C r i t e r i o n A. I t s ho ul d i n s t a n t i a t e pr oc ed ur es t h a t r e l i a b l y l e a dt o v a l i d d e d uc t i on s . O t he r wi s e, it would n o t l e t you r e j e c tin va l id hypo theses , and t h a t i s i t s proposed func t ion .

C r i t e r i o n B. I t s h ou l d be a b l e t o n r e c o g ni z e n h y po t h es e s ( i n o rou t o f consc iousnes s ) , and upon r ecogn iz ing them, co r r ec t l y


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pr oce ss them. Th is i s because i t s domain i s the un ive r s e o fpos s ib le hypo the s es .

C r i t e r i o n C. I t should p rocess hypotheses qu ick ly , au tomat ica l ly ,a nd wi thou t a c ons cious a t t e n t io n . a T he re a r e an i n n i t enumber of ways of carv ing th e world i n t o pr op er t i es , andth e re fo re an in f i n i t e number of r e l a t i on s between p ro pe r t i e st o s er ve a s hypotheses; on average, an enormous number ofhypotheses w i l l have t o be t e s t e d be fo re the c o r re c t one i sh i t upon and th e s imp le s t ge ne ra l i z a t i on made ( t h i s po in t maybe f a t a l t o t h e e n t i r e l e ar n i ng - as - hy p ot h es i s- t e st i n g v i ew ) .Therefo re , pro cess ing must be quick and automatic .

C r i t e r i o n D. I t should deve lop wi thout any sp ec ia l teach ing .Adu l t s r a re ly s i t down and teach children the canons offo rma l log i c , ye t c h i ld re n l e a rn th ings c ons ta n t l y . Th i s meansone of two thi ngs : e i t he r deduction i s not necessa ry fo r mostl e a r n ing , o r th e lo g i c module (o r a " log i c module a c qu i s i t i ondevice"! ) i s inna te . I f the lo g i c module i s necessa ry fo rl e a r n i n g , t h e n it i t se l f cannot be lea rned (Johnson-La ird ,1982 ). Hence, su ppo rt er s of a hypo the sis- tes t ing view ofle a rn i ng a r e c ommi tt ed t o a n inna te ne s s pos i t i on (whethe r theyr e a l i z e it o r n o t ) .

C r i t e r i o n E. With re s pe c t t o the p ropos i t iona l c on te n t of thehypotheses it processes (what P and Q s t an d f o r i n " I f P thenQ") , i t should be content-independent . Because t h i s mechanismi s s uppose d t o a c coun t f o r l e a r n ing i n a l l domains, the domainfrom which th e p ropo s i t ion a l con ten t o f the hypoth es is i staken should have no e f f e c t on how quickl y t he d eductio n i s

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made o r how l i k e l y i t i s t o b e v al i d .

The " do c t r ine of men ta l l og i c " was i n sp i r ed p r imar i l y by c r i t e r i aA and E. T h e p r o p o s i t i o n a l c a l c u l u s i s a s ys te m of r u l e s f o r t h ed e r i v a t i o n o f v a l i d i n f e r e n c e s f ro m p r o p o s i t i o n s l i n k e d byl o g i c a l c o n n ec t iv e s l i k e m , nnf, and QL. l o g i c modulei n s t a n t i a t i n g t h e p r o p o s it i o n a l c a l c u l u s would t h e r e f o r e s a t i s f yc r i t e r i o n A, t h a t t h e module i n s t a n t i a t e pr oc ed ur es t h a t r e l i a b l yl e a d t o v a l i d d e du c ti on s . I n a d d i t i o n , c o nc l us i o ns d e r i v ed v i at h e p r o p o s it i o n a l c a l c u l us a r e v a l i d r e g a r d l e s s of t h e s p e c i f i ccon ten t of t he p ropos i t i ons i nvo lved . I t s r u l e s depend only ont h e t r u t h v a l u e s a ss i g n e d t o t h e p r o p o s i t io n s (whether t h ei n d i v i d u a l p r o p o si t i o n s a r e c o n si d er ed t r u e o r f a l s e ) a nd ont h e i r p o s i t i o n w i t h r e s p e c t t o t h e l o g i c a l c o nn ec t iv es . Forex amp le , t h e " t r u t h t a b l e s n a s s o c i a t e d w i th a c o n d i t i o n a ls t a t e m e n t ( I f P t hen Q ) a nd a b i c o n d i t i o n a l s t a t e m e n t(P i f and on ly i f Q ) a r e :

Cond i t i ona lP Q I f P then Q.....................T T TT F FF T TF F T

Bicond i t i ona lP Q P i f f Q......................

Thus, if P and Q a r e bo th c o n s i de r e d t r u e , t h e n " I f P then Q" i sa l s o c o n s i de r e d t r u e . T h e re f or e , i f P s t a n d s f o r " t he s e a i sblue" and Q st an ds fo r "quantum physi cs i s c o r r e c t " -- and botht h e s e s t a t e me n t s a r e c o n si d er e d t r u e -- t hen th e s t a t emen t " I ft h e s e a i s blue then quantum physics i s c o r r e c t " i s a l s oc o n s i d er e d t r u e . T h i s p r o p e r t y of t h e p r o p o s i t i o n a l c a l c u l u s


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s a t i s f i e s c r i t e r io n E , t h a t t h e l o g i c m od ul e' s r u l e s of i n f e r e n c ebe content- independent .1.3 Do humans h ave a l o g i c module?

Above, I a r gu e d t h a t i f l e a r n i n g o c c u r s t hr ou gh P op pe ri anhyp oth es i s te s t i ng , then humans must have a l o g ic m odule -- apsyc holo gica l mechanism with alg ori thm s t h a t al l ow peop le whoh av e ha d no s p e c i a l t r a i n i n g i n l o g i c t o r e c o g n i ze h yp o th e se s an dd ed uc e o nl y t h e i r v a l i d im p l i c a t i o ns , q u i c k l y , r e l i a b l y , andau tom at i ca l ly . R esea rch on deduc t ive r eason ing ind i ca te s th a thumans have no such ab i l i t y (f or reviews, se e Wason & Johnson-Lai rd, 1972; Johnson-Laird, 1982 ). Because t h e r e i s so l i t t l ed i s s en t on t h i s po in t among psycho log i s t s who s tudy lo g i ca lreasoning, I w i l l c i t e on ly a few i l l u s t r a t i v e examples, drawnfr om t h e l i t e r a t u r e on r e a s o n in g a b ou t c o n d i t i o n a l s t a te m e nt s .

In reas oning about co ndi t io nal s ta te ments , one can make twoc o r r e c t i n e r e n c e s a nd t wo f a l l a c i o u s i n f er ences ( t o conv incey o u r s e l f, i n s pe c t t h e t r u t h t a b l e i n se c t i o n 1 . 2 ) :

C o r r e c t i n f e r e n c e s F a l l a c i o u s i n e rences*A f i mi ng t h e *Denying t h eModus ponens Modus t o l l e n s Consequent Ante ced ent

I f P then Q I f P t hen Q I f P t hen Q I f P t h e QP not-QT h e r e f o r e Q Therefore not-$ Phere fo re P not-PTherefore not-Q* T he se i n f e r e n c e s a r e f a l l a c i o u s b ec au se a c o n d i t i o n a l d oe s n o tc l a i m t h a t P i s t h e only pos s ib l e an teceden t of Q. Consider aconcre t e , c ausa l s t a t em ent : " I f i t r a i n s t h e n t h e g r a s s i s wet."I f i t h as a ai ne d, t h e gr a s s may or may not be wet -- perhapsI have been wat eri ng th e lawn with my sp ri nk le r . To conclude " i tr a ined" (o r "it d i d no t r a i n" ) f rom the r u l e p rem ise and the"grass i s wetn premise i s t o "af f i rm the consequent" . Toconc lude " t he g ra s s i s d r y H ( o r " t h e g r a s s i s wet" ) f rom the ru l epremise and the " i t d i d no t r a in " p remise i s t o "deny the ant ecedent" .No v a l i d in fe re nc e can be drawn from th es e s e t s of premises.


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Minimal ly, a lo g i c module capab le of eva l ua t i ng con di t i ona lhypo the s es s hould i n s ta n t i a t e p roce dure s th a t qu ick ly a ndr e l i a b l y accomplish modus ponens and modus t o l l en s. Furthermore,it should be immune t o th e two fa l la c i ou s in erences. Thea lgor i t hms invo lved a r e s imple and wel l -de f ined -- amicrocomputer can e a s i l y be programmed t o run them.

Moreover, the only f a i r way t o te s t f o r a lo g i c module i s t ous e s t a t e me n ts th a t e xp res s un fa mil i a r r e l a t i ons , s uc h a s " I f a no b j e c t i s a t r i a n g l e , t h en i t i s r e d w , o r " I f t h e r e i s an A onone s i de of t h e c a r d , t h e n t h e r e i s a 3 on th e o the r s i de w. Thel og i c module i s s uppose d t o be ne c es s a ry fo r l e a rn in g , th a t i s ,f o r t h e c o n s t r u c t i o n of m knowledge. I f t h i s i s i t s purpose,then i t s hou ld be good a t ha nd l ing un fa mi l i a r r e l a t io ns .Furthermore , th e use of r e l a t i on s drawn from unf ami lia r domainsprov ides a c leaner exper imenta l des ign . I t p r e v e n t s s u b j e c t sfrom simply "looking up fa c t s " t o answer th e quest i on: one needno t e ngage i n any re a s on ing p roc e s s t o de c ide th a t " A l l swans ar eorange" i s f a l s e . C o nd i ti o na l s r e l a t i n g l e t t e r s t o numbers a r egood candi da t es because l e t t e r s and numbers a r e fam il ia r enoughbut re la t io ns between them a r e no t . For lac k of a be t t e r te rm, Iw i l l f o ll ow t h e l i t e r a t u r e a nd c a l l s uch c o n d i t i o n a l s " a bs t r a ct . "

P r e d i c t i o n A: Valid deduc t ions a r e made f re quen t ly and re l i ab ly .

I tem. Shapir o (r epo rt ed i n Wason & Johnson -Lair d, 1972, pp. 43-44) a s k e d 20 s u b j e c t s t o e v a l u a t e t h e v a l i d i t y of a b s t r a c tve rs ion s of t he fou r in fe r ences l i s t e d above . I f humans have al o gi c module, her su bj ec ts should make few i f any err or s : theys ho ul d ju dg e t h e f i r s t two i n f e r e n ce s v a l i d a nd t h e l a s t two


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inf ere nce s in va l i d . Er ro rs should be randomly di s t r i bu te d amongt h e f o u r i n e rences . Th i s t a s k i s very simple -- i t does noteven req u i r e s ub j ec t s t o genera te conc lus ions themse lves. A l lt hey have t o do i s c o r r e c t l y r e co g ni z e i n f e r e n c e s t h a t h a vea l rea dy been made a s va l id o r inva l id .

The er r o r r a t e was reaso nably low fo r modus ponens (5%), u tth e e r r or ra t e was 52.5% fo r modus to l l en s , 20% fo r a f f i rmi ng t h econsequent , and 25% fo r denying th e antecede nt . Half t he t imes u b j e c t s were j udging a va l i d in fe rence inva l i d , and a qua r t e r o fth e tim e they were judging inv a l i d in fe rences va l id . Er r o r s werenot randomly d is t r i b u te d among th e fo ur con dit ion s. Thed i s t r i b u t i o n o f e r r o r s i n d i c a t e s t h a t s u b j e ct s f i n d i tp a r t i c u l a r l y d i f f i c u l t t o r ec og ni ze t h e v a l i d i t y of modus t o l l e n s .

Item. In an experiment by Gibbs (r ep or te d i n Wason & Johnson-Lai rd , 1972, p. 57-59) su bj ec ts had t o gener a te deduct ions . Onaverage, 4 4 % of th e problems req ui ri ng t h e use of modus ponenswere done i nc or re c t ly , and 80% of those re qu i r in g modus t o l le n swere done inc or r ec t ly . I n bo th cases , i nco r re c t i n fe re ncescorresponded t o comm it ting the f a l l ac y of a f f i rm ing t heconsequent. Modus ponens was co rr ec tl y used 2.8 t ime s a s of te na s modus t o l l e n s was.

I tem. Mazzocco (r ep or te d i n Legrenzi , 1970) found t h a t s ub je ct ser roneously assume th a t " I f P then Q" i s e q ui v a le n t t o " I f Q thenP" when t h i s makes a problem eas ie r t o "sol ven. Po l l ar d & Evans(1 980 ) f ou nd t h a t s u b j e c t s f r e q u e n t l y view l o g i c a l l y d i s t i n c tcon d i t ion a l s a s im ply ing one ano the r .

I tem. Pol lard & Evans (1981) found th a t s ub je ct s have a15


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pronounced t endency t o judge an in fer en ce va l i d when they agreewi th t he con clus io n , and inv al id when they do not ag ree wi th t heconc l us i on -- r e g a r d l e s s o f i t s t r u e v a l i d i t y .

The c l a i m t h a t humans have a qu i ck and r e l i a b l e dedu c t i vecomponent seems t o f a l l beo r e i t t a k e s i t s f i r s t s te p.E x pe ri m en ta l r e s u l t s do n o t e ve n s u p po r t c r i t e r i o n A , t h a t p e o p l ebe a b l e t o r e l i ab l y make va l i d deduc ti ons . A lt hough peop l e havesome measure of suc ces s i n r ecogni z ing th e va l i d i t y of modusponens, t hey a r e no t good a t u s i ng i t t o g e n e r a t e d e d u ct i on s .They a r e q u i t e s u s c e p t i b l e t o m aking f a l l a c i o u s i n e r e n c e s, andt hey seem t o l ac k a p r ocedur e co r r e s pond i ng t o modus t o l l e n sa lm o st e n t i r e l y . The l i t e r a t u r e on l o g i c a l r ea so n in g i s q u i t eco ns i s t en t on t h i s po i n t . Accor di ng t o J ohnson-La ir d ( 1982) ," t h e d o c t r i n e of l o g i c a l i n a l l i b i l i t y i s e i t h e r f a l s i f i e d by t h er e s u l t s o f some e x pe ri m en ts on s y l l o g i s t i c re a s o n i n g o r e l s eempi r i ca l l y vacuous . "

To s a v e t h i s p e r s p e c t i v e , one m ig ht a r g u e t h a t t h e l o g i cmodule has a s i mpl e r des i gn and a more s pe c i f i c f un c t i on .Perhaps i t does no t have p r ocedur es f o r de r i v i ng deduc t i v ei m pl i ca t io n s a t a l l : p erh ap s it c an o nl y lo ok f o r f a l s i f i c a t i o n .Nothing coul d be s impler t o program. Consider any hy po th es is oft h e f or m, " I f P t hen Q." The t r u t h t a b l e f o r t h e c o n d i t io n a lshows t h a t t h e r e i s o nl y one c i rc u ms ta n ce t h a t c an f a l s i f y t h i shypo thes i s : t he co-occurrence of P and not-Q. A l o g i c modulec a p a b l e o nl y of f a l s i f i c a t i o n would s c an a l l i n s t a n c e s of P anda l l i n s t a n c e s o f no t-Q . I t would r e j e c t t h e hypo t hes i s i f any Pwas pa i r ed wi th a not-Q or i f any not-Q was pa i r ed wi th a P.


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The Wason s e l e c t i o n t a s k t e s t s t h i s p r e d i c t i o n . P e t e r Wasonwas i n t e r e s t e d i n P o p pe r 's view t h a t t h e s t r u c t u r e of s c i e n c e washypo t het i co -deduc t ive . The se l e c t i o n t a s k a l l ows one t o seewhether p eo pl e r e a l l y a r e f a l s i f i c a t i o n i s t s i n t e s t i n gh yp ot he se s. I n t h e s e l e c t i o n t a s k , a s u b j e c t a sk ed t o t e s t ahypo t h es i s o f t he f orm " I f P t hen Q" w i th r e s p e c t t o a u n iv e rs eo f f ou r ca r ds r e p r es en t i ng po ss i b l e pa i r i ng s of P and not -P w i t hQ and not-Q. Here i s t he o r i g i n a l s e l ec t i on t a s k (Wason, 1966):

Cons ide r t he f o l l owi ng sen t ence :" I f a ca rd has a vowel on one s i de then i t ha s an ev en number ont h e ot h e r s i d e . "I t r e f e r s t o t h e s e f o u r c ar ds :

Each ca rd has a l e t t e r on one s i d e and a number on the oth ers i de . Name tho se car ds , and only those ca rds , which need t o bet u r ned over i n o rde r t o de t e rmi ne dec i s i v e l y whe ther t he sen t encei s t r u e or f a l s e .

The card s were r e a l cards , and an exper imenter adm ini s t e re d th et a s k i n pe rs on t o one s u b j e c t a t a t im e.

The Wason se l e c t i o n t a s k has a gen e r a l so l u t i o n . Tur n ove ra l l c a r d s d i s p l a y i n g P ( t o s e e i f t he y h ave a n ot-Q on t h e o t h e rs i d e ) a nd t u r n o ve r a l l c a r d s d is p l a y i n g n ot -Q ( t o s e e i f t h eyh av e a P on t h e o t h e r s i d e ) . T he re i s no p o i n t i n t u r n i n g o v er Qor not -P , because any va lue on th e o ther s id e of the se card swould be co ns i s t en t w i t h t h e hypo t hesi s .

T h i s p r o v id e s a d i r e c t t e s t o f t h e m o di fi ed view of t h el o g i c m od ul e' s f u n c t i o n . I f t h e l o g i c module i s s p e c i a l i z e d f o rt e s t i n g hy p ot he s es th ro ug h d e d u ct i v e f a l s i f i c a t i o n , t he n s u b j e c t s


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shou l d i mmed ia t el y r e a l i z e t h a t t hey must t u r n ove r t h e E c a r d(P) and t h e 7 ca r d ( no t- Q) .

They do no t. On av er ag e, on ly 4 t o 1 0 p e rc e nt of a l ls u b j e c t s c h o o s e P and not -Q when confr ont ed wi th an a b s t ra c thy po th es is (Wason, 198 3) . The ma jo r i t y pick only PI o r P and Q ,a s i f t he y a r e t r y i n g t o co nf irm t h e e x i s t e n c e of a r e l a t i o n ,r a t h e r t h a n f a l s i f y a p ro po se d r e l a t i o n . T h i s r e s u l t h a s beenr e p l i c a t ed many t i mes , under a w ide va r i e t y of cond i t i o ns : w i t hd i f f e r e n t a b s t r a c t p r o p os i ti o ns s ta nd in g i n f o r P and Q, withv a r i a t i o n s i n t h e l i n g u i s t i c fo rm at of t h e h yp o th e si s , w it hv a r i a t i o n s i n how t h e i n o rm a ti on i s r e p r e s e n t e d on t h e c a r d s ,w i t h va r i a t i o ns i n t h e i n s t r uc t i on s ( e .g ., Wason, 1968; Wason,1969a & b; Wason & Joh nso n-L aird , 1970; Wason & Shapi ro , 1971;Goodwin & Wason, 19 72 ; Wason & Golding , 1974) .

Fur thermore, it i s v er y d i f f i c u l t t o t e ac h p eo pl e t h es o l u t i o n . A w id e v a r i e t y of " t h e r a p i e s * h av e be en t r i e d ; t h e yhave r e s u l t e d i n l i t t l e or no f a c i l i t a t i o n i n f a l s i f i c a t i o nra te s . For example :

1. For eac h of 24 sample ca rd s, su bj ec ts were asked whether orno t each ca r d was con s i s t en t w it h t h e r u l e ; t hey were g i venf eedback abou t t h e i r answers . They were t hen a sked t o so l v e as e l e c t i o n t a s k u s i n g t h e s ame r u l e (Wason & Shap i r o , 1971) .

2. For each of 24 sample ca rds , su bj ec t s were asked t o imaginea v a l u e on t h e o t h e r s i d e of t h e c a r d t h a t would f a l s i f y orv e r i f y t h e r u l e ; t h e y were g i ve n fe ed b ac k a b o u t t h e i r a ns we rs .They w ere t h e n a s k ed t o s o l v e a s e l e c t i o n t a s k u s i n g t h e samer u l e (Wason & Shap i r o , 1971) .


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3 . Sub j ec t s were a l l owed t o t u r n over t he c a r ds t hey hads e l e c t e d , a sk ed w heth er e ac h v e r i f i e d o r f a l s i f i e d t h e r u l e ,and co r r ec te d i f wrong; then they were r e t e s t ed (Hughes, 1966) .

4 . A d u p l i c a t e s e t of f u l l y r e v ea l ed c a r d s were p r e s e nt f o rsu b j e c t s t o i n s pe c t (Goodwin & Wason, 1972).

Even p rof es s io na l lo g i c i an s have been known t o ge t t he problemwrong1 (Wason & Johnson-Laird, 1972, p. 179)

Su bj ec t s ' per formance on the Wason se le c t io n t as k i s t h emost damning ev idence ag ai ns t the l earn ing-as -hypot hes i s - t e s t ingv iew. Confr on ted w i t h a novel hypo t hes i s , su b j e c t s do no t t r y t of a l s i f y it. Yet t h i s i s a pa r ad i gmat ic case i n wh ich t hey shou l du se d ed uc ti ve f a l s i f i c a t i o n . T hi s r e s u l t f a l s i f i e s t h e m od if i edview of t h e l o g i c module a s s p e c i a l i z e d f o r s p o t t i n g f a l s i f y i n gevide nce. I n ad di t i on , because modus ponens and modus t o l l e n sc an be us ed t o s o l ve t h e s e l e c t i o n t a s k , t h i s r e s u l t , l i k e t h ep r e v io u s l y c i t e d e v id e nc e , f a l s i f i e s t h e o r i g i n a l view of t h el o g i c module a s i n s t a n t i a t i n g d e d u c t i v e p ro c ed u re s t o b e u s ed i nf a l s i f y i n g h yp ot he se s.

Beat ing a dead horse.A l o g i c modu le necessa r y f o r l ea r n i ng shou l d mee t f ou r o t he r

c r i t e r i a ( B - E ) , b u t t h e s e a r e p r e d i c a te d on i t f u l f i l l i n gp r e d i c t i o n A -- t h a t p e o p l e f r e q u e n t l y a nd r e l i a b l y make v a l i ddeduc t i ons . P r ed i c t i on A ha s been shown t o be f a l s e , sot e c h n i c a l l y , t h e o th e r p re d i c t i o n s f a l l w ith i t . J u s t t o bethorough, however, I would l i k e t o b r i e f l y d i s c u s s e a chs e p a r a t e l y .


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P r e d i c t i o n B: The l o g i c module can "r ecogn izeu hypotheses ,and upon re co gn iz in g them, pr oc es s them.Hypotheses about t he world do not come i n j us t one

l i n g u i s t i c fo rma t. A l o g i c module shou ld be a b l e t o r ecogn izea nd o p e r a t e on t h e l o g i c a l " de ep s t r u c t u r e u of a h y p o t h es i s ,p roduc ing va l i d deduct ions r ega rd l e s s of i t s l i n g u i s t i c for mat .The amount of t ime th e convers ion t o deep s t ru c t ur e ta ke s mayd i f f e r w i th l i n g u i s t i c fo rm at , n o t t h e v a l i d i t y of t h e de d uc ti o nsmade,

T h i s i s n o t t h e c a s e . A number of s t u d i e s show t h a t (1 )d i f f e r e n t l i n g u i s t i c f o r m at s of t h e same h y p o t h es i s d i f f e r i n howl i k e l y t he y a r e t o e l i c i t a v a l i d d e du c t io n , a nd ( 2 ) a l i n g u i s t i cformat th a t f a c i l i t a t e s deduc t ion fo r one problem may impedede du ct io n i n an ot he r (e.g., Van Duyne, 1974; Roberge, 1978, 1982;Bracewell & H i di , 1 9 7 4) . S u b j e ct s i n t h e s e s t u d i e s h ad no t i meco ns t r a in t s , so d i f f e r enc es i n per fo rmance can be accounted fo ro nl y by d i f f e r e n c e s i n l i n g u i s t i c fo rm at .

P r e d i c t i o n C: Val id deduc t ions a r e made quick ly , au tom at ica l ly ,and wi thou t consc ious a t t en t i on .I n t h e e xp e ri m en t s c i t e d u nder p r e d i c t i o n A, s u b j e c t s w ere

p e r m i tt e d t o d e vo te a l l t h e t i me and c on sc io us a t t e n t i o n t o t h eproblem t h a t they wanted , ye t they s t i l l di d no t make va l i ddeduc t ions . Clea r l y t hey do no t make va l i d deduc t ions qu i ck ly ,au tom a t i ca l ly , and wi thou t consc ious a t t e n t on .

P r e d i c t i o n D: The l o g i c module deve lops wi thout any sp ec ia lteaching .A ga in , t h e e v i de n ce c i t e d f o r p r e d i c t i o n A shows t h a t people

d o n o t r e l i a b l y make v a l i d d e d u ct i o ns w it h o ut s p e c i a l t r a i n i n g .Indeed, i t i s n o t c l e a r t h a t t h e y r e l i a b l y make v a l i d d e du c t io n s

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e ven w i th s p e c i a l t r a i n i n g . A s the the rapy exper imen ts showed ,p e rfo rm a n ce on t h e Wason s e l e c t i o n t a s k p ro ve d r e l a t i v e l yi m pe rv io us t o s p e c i a l t r a i n i n g t e c h ni q u e s, and ev en p r o f e s s i o n a ll o g i c i a n s f i n d i t d i f f i c u l t .P r e d i c t i o n E t The l o g i c module i s content- independent .

W ason's f i r s t s e l e c t i o n t a s k s u se d hy po th es es t h a t e x pr e ss e da b s t r a c t r e l a t i o n s , u s u a l l y i n v o l vi n g l e t t e r s a nd numbers.Perfor mance was uni for mly poor. However, a number of experimentsi n t h e e a r l y 1 9 70 ' s r e ve r se d t h i s r e s u l t (Wason & Shap i ro , 1971 ;Johnson-Laird , Legrenz i & Legrenz i, 1972; Bracewell & Hidi , 1974;G i lh o o ly & F a lc o n e r , 1 9 7 4 ) . The se e x p e r im e n t s s u g g e s t e d t h a t i ft h e c o n t e n t of t h e r u l e b ei ng t e s t e d e x p r es s e s a " f a m i l i a r , "" r e a l i s t i c , " o r " t h e m at i c " r e l a t i o n , s u b j e c t s do r e as o nl o g i c a l l y on t h e s e l e c t i o n t a s k . T h i s e nhan cem ent of l o g i c a lp er fo rm a nc e w i t h f a m i l i a r m a t e r i a l s i s known a s th e "conte nte f f e c t " o r t h e " t h e m at i c m a t e r i a l s " e f f e c t on t h e Wason s e l e c t i o nt a s k .

I n i t i a l l y , r e se a rc h er s th ou gh t t h a t t h e f a m i l i a r i t y orr e a l i s m of t h e m a t i c c o n t e n t somehow f a c i l i t a t e s t h e u se ofd e d u c t i v e l o g i c (Wason & Sh ap iro , 1971; Johnson-Laird, Legrenz i &Legrenz i , 1972). The p ro bl em w i th t h i s e x p l a n a t i o n i s t h a t t h ephenomenon i s q u i t e d i f f i c u l t t o r e p l i c a t e . Some f a m i l i a rc o nt e nt seems t o f a c i l i t a t e t h e us e of d e du c ti v e l o g i c ; o t h e rf a m i l i a r con te nt does no t (e.g. , Van Duyne, 1976; Manktelow &Evans, 1979; Gr ig gs & Cox, 1982; Cox & Grigg s , 1982; Reich &Ruth, 1982; Yachanin & Tweney, 1982; G ri gg s & Cox, 19 83 ). I na d d i t i o n , t h e same f a m i l i a r c o n t en t s eems t o f a c i l i t a t e l o g i c a tsome t e s t i n g l o c a t i o n s , b u t n o t a t o t h e r s ( e .g ., G ol di ng , 1 98 1;

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Griggs & Cox, 1982; Yachanin & Tweney, 198 2). Th is sho uld no th ap pe n i f f a m i l i a r c o n t e n t s im pl y a c t i v a t e s a l o g i c module.

Whatever t he exp l ana t i on , t he cog n i t i v e p r oces s es t h a tg o ve rn r e a s on i n g a b o u t l o g i c a l c o n d i t i o n a l s i n t h e Wasons e l e c t i o n t a s k a r e c l e a r l y n o t c on te nt -i nd e pe n de n t.

The hypo thes i s th a t humans have the s o r t of lo g ic modulen ec e ss ar y f o r ~ o p p e r i a n - s t y l ee ve ry da y l e a r n i n g f a l t e r e d be o r et a k i n g i t s f i r s t s t ep . Not one of t h e f i v e d e f i n i n g c r i t e r i a ofa l og ic module i s f u l f i l l e d by t h e r e s u l t s of e x pe ri me n ts onl o g i c a l r e a s o n i n g .

T h i s r a i s e s some s e r i o u s q u e s t i o n s : I f p e o p l e a r e n o t u s in gd e d u c t i v e r u l e s t o r e a so n a b ou t c o n d i t i o n a l s t a t e m e n t s , t h en whatr u l e s a r e t h e y us i ng ? And i f p e o p l e a r e n o t l e a r n i n g v i aPopper i an hypo t hes i s t e s t i n g , t hen how a r e t hey l ea r n i n g?

The d i scov ery of t he con tent e f f e c t on the Wason se l ec t i ont a s k r a i s e s t h e p o s s i b i l i t y t h a t r ea so ni ng a bo ut l o g i c a lc o n d i t i o n a l s i s governed by content -dependent co gn i t iv ep r oc e ss e s. I nd ee d, a f t e r y e a r s s pe n t r e s e ar c h i ng t h i s e f f e c t ,Wason and Johnson-Laird commented t h a t th e co nd it io na l " i s n o t acr ea tu re of con s tan t hue, but chameleon-l ike , t ak es on th e co lourof i t s sur roundings : iJ s b det- j;a some bykhe p r o ~ o s i t i o n s& connects" (1972, p.92, i t a l i c s t h e i r s ) .They s ay t h a t t he p r i nc i p l e s gove r ni ng t he "cohesi on o fd i sc our se" probably hold the key to i t s many meanings, and t h a t" t h e n a t u r e of t h e s e p r i n c i p l e s i s l i t t l e u nde rs too d -- t heyp ro b ab ly i n v o l v e more t h a n p u re l y l i n g u i s t i c f a c t o r s . " Thei n v e s t i g a t i o n o f Darwinian a l g o ri t h m s p r e s en t e d i n t h i s t h e s i s i sa pr e l im inar y enq ui ry i n t o what "moren they involve .

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