An explanation of Aquinas' Second Way of proving the existence of God. NOTE: This article requires you to be familiar with the First Way.
I. Essence vs Existence
An "essence" is the attribute or set of attributes that make an object or substance what it fundamentally is, and which it has by necessity, and without which it loses its identity. An example would be Santa Claus, who delivers toys on Christmas in a flying sleigh. Without those properties, he isn't Santa Claus.
However, whether he actually exists or not is another story. "Existence" is not one of Santa Claus' necessary properties to be what he is rather than some other thing. He could either exist, or not. The same goes for most objects in our experience.
II. Existence: Internal or External?
The argument is trying to answer the question, "Why do things exist now, and what keeps them in existence now?" Since most things in our experience do not have "existence" as part of their essence, then their existence is separate from their essence and must come from a source other than themselves.
III. An Essentially Ordered Series, Again
As in the First Way, we again trace "down", in the present, in an essentially ordered series. This argument works just like the First Way, with "essence" being the potential, and "existence" being the actual.
If a thing's existence comes from a source other than itself, and the same thing applies again to that next thing and so on, then again the chain must terminate in a member that "holds up the whole stack", and this can only be something whose essence is identical to its existence: just pure existence itself. Something that is fully actual, and thus has the same attributes and is therefore the same agent as in the First Way.