Tzaddikim Do Not Die: They Transform the Physical into Spiritual

"And Sara died" (Chayei Sara 23:2)

The verse hints at what it says "Kiryas Arba" ("City of the Four"), to say that G-d-forbid one should not connect death with tzaddikim. Rather, Kiryas Arba means the city that was built from the four yesodos (fundemental elements) became separated and departed from them. And in saying "this is Chevron" implies that even though she died, she caused them [the four yesodos] to have the aspect of unification (chibur) because tzaddikim, even though they will die yet the body is not negated from life because tzaddikim in their deaths are called living. Go and learn from what occurred with Rav Achai bar Yoshiya (Shabbos 152).

The reason is because when they are in the world they transform the aspect of holy spirituality that is in their nefesh via their good deeds and abundant Torah that they accomplish in this world. Go and learn from the Rambam (Hilchos Yesodei Hatorah perek daled) that it is a rule of Hashem that one yesod can transform into its neighboring yesod: earth transforming into water, and similarly we find that via the clinging of a person to his Creator he can transform all the yesodos to [the highest] yesod of fire, and from the [material] yesod of fire to the yesod of fire of the neshoma. (Ohr Hachayim on the posuk)