What sustains, here and now, the existence and operation of all that is? Not when the universe began, but what grounds its causal activity at each instant. Recovering Aquinas's Second Way in a synchronic key, this book argues that every network of causes acting at once and hierarchically must be finite and rest on a necessary, unconditioned first cause, the pure act of being. In dialogue with contemporary physics, it offers three convergent natural-deduction proofs, reinforced by modal reasoning in S5, and answers the strongest objections, from Hume and Russell to quantum indeterminism, the multiverse, and the recent thesis of existential inertia. A rigorous, contemporary case that the intelligibility of the real points to a transcendent foundation.