The entry fas comes from Latin roots, specifically from the voice “fas” that refers to just, licit; which is the opposite of the word nefas which means unfair and illicit. As the real Spanish academy expresses it, colloquially the expression “por fas o por nefas” is used, this means “for one thing or another”, also on the other hand it can mean "justly or unjustly". Other sources state that the etymology of the word fas comes from Sanskrit and that it refers to the expression of the will of the gods, what is just and allowed by them; and the word nefas were those certain customs that could not be followed or carried out due to anxiety or fear of the anger and revenge of said gods. Both terms suggested the first group of norms that dominated the coexistence of organizations prior to the civitas or the resident citizenship of Rome.
In ancient Rome, fas was understood to be the norms emanated by the divinity or gods, which at the beginning used to be confused with the ius, when the organized society was under the dominions of religion, to later differentiate them from it, in a process of transformation that began after the first written law ruled in Ancient Rome, this was the law of the XII Tables, leaving human law to be a secondary source of law; since until then the elections had not carried out a significant legislative task.
On the other hand, in the archaic era, the duplicity between Ius and Fas is manifested, although it should be noted in the beginning both concepts were closely linked. the difference between these is that Ius was fair and Fas was lawful, these two words were used at this time as adjectives. Then finally it can be said that fas is the divine condition of the lawfulness of a behavior.
