The second of Thomas Aquinas's five ways argues that there is a regress of essentially-ordered causes. By "essentially," he means what we would call "sustaining," much like in the argument from motion. Unfortunately, Thomas leaves much of the second way open to interpretation. He unambiguously states that a regress of sustaining causes cannot be infinite, and so he concludes that there must exist an Uncaused Cause. However, this particular formulation of the argument is susceptible to the charge of committing the taxicab fallacy.
The argument from motion (the first way) avoids this by stating that everything in motion has its motion sustained by another. Since the regress of sustaining movers cannot be infinite, there must be an Unmoved Mover. The first way cannot be charged with committing the taxicab fallacy, since the Unmoved Mover is not in motion and thus the causal principle is inapplicable to it.
One way of clarifying the second way is to supplement it with the Principle of Sufficient Reason (PSR). The argument would then look like this:
1. Contingent things exist. (Premise)
2. Every contingent thing has a sustaining cause. (Premise)
3. Either an Uncaused Cause exists, or else there is an infinite regress of contingent sustaining causes. (Implied by 1 and 2)
4. There cannot be an infinite regress of contingent sustaining causes. (Premise)
5. Therefore, an Uncaused Cause exists. (From 3 and 4)
In order to bridge the gap between Uncaused Cause and God, one would then need to introduce the metaphysics of De Ente et Essentia ("On Being and Essence").