More generally, reason in the strict sense requires the ability to create and manipulate a system of symbols, as well as indices and icons, according to Charles Sanders Peirce, the symbols having only a nominal, though habitual, connection to either smoke or fire.
Mere overordnet kræver fornuft i streng forstand evnen til at skabe og manipulere et system af symboler og, ifølge Charles Sanders Peirce, herudover indekser og ikoner, da symbolerne kun har en nominel, omend tilvænnet, forbindelse til enten røg eller ild.