"It is necessary to posit something which is necessary of itself, and has no cause of its necessity outside of itself but is the cause of necessity in other things. And all people call this thing God"

Thomas Aquinas, a renowned medieval thinker and theologian, is frequently celebrated for his Five Ways, which are arguments for the existence of God. This quote is drawn from his third way, the argument from contingency and requirement, found in his critical work, "Summa Theologica". The main idea of this quote revolves around the principle of essential and contingent beings.

Aquinas argues that on the planet, we observe things that are contingent, meaning they are capable of existing or not existing. These things have causes and their presence is not needed-- they come into being and die. If everything were contingent, then, in theory, there might have been a time when absolutely nothing existed. However, if that held true, absolutely nothing would exist now, because something can not originate from absolutely nothing. For that reason, there must be something that exists necessarily-- that did not originate from something else-- that has constantly existed.

The quote posits the presence of a being whose need is intrinsic, not depending on any other being or cause. This being is a necessary being that is self-existent and the reason contingent beings exist. Aquinas determines this self-existent essential being as God. This necessitates that God's existence does not rely on anything else however is the grounding for the existence of everything else, basically functioning as the main reason for existence as such.

By saying "all people call this thing God", Aquinas is asserting that throughout human cultures and philosophies, this fundamental principle of a first cause or needed being is recognized and is given the name "God". God, therefore, is understood as the self-existent foundation that imparts existence to whatever else, ensuring their continued presence and ordering the universe. This philosophical reasoning intends to bridge human logic with theological understanding, offering a reasonable basis for belief in a divine creator.