(a distillation of ST. 1.5.1)
The good is whatever a thing desires or tends to or seeks to preserve
(Since we desire, tend to, or seek to preserve either a real or apparent good, but no one tends to the apparent good as apparent. Therefore, taken precisely, the good is the whatever one desires, tends to, etc.)
But whatever exists desires or tends to or seeks to preserve its own existence.
(This is evident in the living and the organic. In the non-living it is clear from the stability of atomic structure, or at least in what is elemental in what is called the elements, since what is simplest in the inorganic world must be conserved though all change. That said, this does not require that everything that exists seeks to preserve itself in every circumstance, only that the exceptions are confirmations of transcendental goodness. See the corollary.)
Therefore the existence of whatever exists is good.
Cor. 1: Since there are higher and lower sorts of existence, then if a thing is not the highest thing it is not the highest good. This opens the possibility that it might have to give up its life for the sake of something greater (the lower always carries the possibility of being ennobled through sacrifice.)