Validity and Invalidity, Soundness and Unsoundness

Valid: an argument is valid if and only if it is necessary that if all of the premises are true, then the conclusion is true; if all the premises are true, then ... MainPage PhilosophicalTerms Reconstructing anArgument ShortListofDefinitions GlancesAhead RotatingValidity Exercises                             ValidityandInvalidity,SoundnessandUnsoundness  Thetaskofanargumentistoprovidestatements(premises)thatgive evidencefortheconclusion. Therearetwobasickindsofarguments.     Deductiveargument:involvestheclaimthatthetruth ofitspremisesguaranteesthetruthofitsconclusion;thetermsvalid andinvalidareusedtocharacterizedeductivearguments. Adeductive argumentsucceedswhen,ifyouaccepttheevidenceastrue(thepremises), youmustaccepttheconclusion. Inductiveargument:involvestheclaimthatthetruth ofitspremisesprovidessomegroundsforitsconclusionormakesthe conclusionmoreprobable;thetermsvalidandinvalidcannotbeapplied. Valid:anargumentisvalidifandonlyifitisnecessary thatifallofthepremisesaretrue,thentheconclusionistrue;if allthepremisesaretrue,thentheconclusionmustbetrue;itisimpossible thatallthepremisesaretrueandtheconclusionisfalse. Invalid:anargumentthatisnotvalid. Wecan testforinvaliditybyassumingthatallthepremisesaretrueandseeing whetheritisstillpossiblefortheconclusiontobefalse. If thisispossible,theargumentisinvalid.       Validityandinvalidityapplyonly toarguments,notstatements. Forourpurposes,itisjustnonsense tocallastatementvalidorinvalid. Trueandfalseapplyonly tostatements,notarguments. Forourpurposes,itisjustnonsense tocallanargumenttrueorfalse.Alldeductiveargumentsaspiretovalidity.       Ifyouconsiderthedefinitionsof validityandinvaliditycarefully,you'llnotethatvalidargumentshave thefollowingimportantproperty: validargumentspreservetruth.  Ifallyourpremisesaretrueandyoumakeavalidargumentfromthem, itmustbethecasethatwhateverconclusionyouobtainistrue.  (Weshallseebelow,however,thatvalidargumentsdonotnecessarily preservetruthvalue: itisentirelypossibletoarguevalidlyfrom falsepremisestoatrueconclusion). Sound:anargumentissoundifandonlyifitisvalid andcontainsonlytruepremises. Unsound:anargumentthatisnotsound. Counterexample:anexamplewhichcontradictssomestatement orargument(ex.acounterexampletothestatement“Allfifteen year-oldshavebluehair”wouldbeafifteen-year-oldwithoutblue hair);foranargument,acounterexamplewouldbeasituationinwhich thepremisesoftheargumentaretrueandtheconclusionisfalse;counterexamples showstatementstobefalseandargumentstobeinvalid.  IV.FormsofArgument