Sofar，then，ashasyetbeenstated，therelationsbetweenthesetwoordersofpartsaredeterminedbyafinalcause。Wehave，however，toinquirewhethernecessitymaynotalsohaveashareinthematter；anditmustbeadmittedthatthesemutualrelationscouldnotfromtheverybeginninghavepossiblybeenotherthantheyare。
Forheterogeneouspartscanbemadeupoutofhomogeneousparts，eitherfromapluralityofthem，orfromasingleone，asisthecasewithsomeoftheviscerawhich，varyinginconfiguration，areyet，tospeakbroadly，formedfromasinglehomogeneoussubstance；butthathomogeneoussubstancesshouldbeformedoutofacombinationofheterogeneouspartsisclearlyanimpossibility。Forthesecauses，then，somepartsofanimalsaresimpleandhomogeneous，whileothersarecompositeandheterogeneous；anddividingthepartsintotheactiveorexecutiveandthesensitive，eachoneoftheformeris，asbeforesaid，heterogeneous，andeachoneofthelatterhomogeneous。
Foritisinhomogeneouspartsalonethatsensationcanoccur，asthefollowingconsiderationsshow。
Eachsenseisconfinedtoasingleorderofsensibles，anditsorganmustbesuchastoadmittheactionofthatkindororder。Butitisonlythatwhichisendowedwithapropertyinpossethatisactedonbythatwhichhasthelikepropertyinesse，sothatthetwoarethesameinkind，andifthelatterissinglesoalsoistheformer。
Thusitisthatwhilenophysiologistseverdreamofsayingofthehandorfaceorothersuchpartthatoneisearth，anotherwater，anotherfire，theycoupleeachseparatesense-organwithaseparateelement，assertingthisonetobeairandthatothertobefire。