Aquinas' Third Way of proving the existence of God. NOTE: You must be familiar with both the First Way and the Second Way in order to understand this argument.
I. Things Coming and Going
We see around us that things come into and go out of existence. People are born, and people die. Rivers run, and they dry up. Most things around us are transient, and not permanent, and thus it is possible for them not to exist.
II. Given Infinite Time...
If it is possible for the things around us to not exist, then it is also possible that they all not exist at once. Given infinite past time, this possibility would eventually be realized and nothing would now exist. However, if nothing existed at some point in the past, then nothing would now exist, which is obviously false.
III. A Necessary Being
Therefore, there must be at least one thing that cannot be created or destroyed. The most obvious candidate for this is matter/energy.
This incorruptible thing, in this case matter/energy, either has existence as part of its essence, or not. If not, then the argument proceeds into the Second Way.