Proclus' Elements of Theology:
Proposition 11. All that exists proceeds from a single first cause.
For otherwise all things are uncaused; or else the sum of existence is limited, and there is a circuit of causation within the sum; or else there will be regress to infinity, cause lying behind cause, so that the positing of prior causes will never cease. But if all things were uncaused, there would be no sequence of primary and secondary, perfecting and perfected, regulative and regulated, generative and generated, active and passive, and all things would be unknowable. For the task of science is the recognition of causes, and only when we recognize the causes of things do we say that we know them.
And if causes transmit themselves in a circuit, the same things will at once prior and consequent; that is, since every productive causes is superior to its product (prop. 7), each will be at once more efficient than the rest and less efficient. (It is indifferent wether we make the connexion of cause and effect and derive the one from the other through a greater or a less number of intermediate causes; for the cause of all these intermediaries will be superior to all of them, and the greater their number, the greater the efficency of that cause.)
And if the accumulation of causes may be continued to infinity, cause behind cause for ever, thus again all things will be unknowable. For nothing infinite can be apprehended; and the causes being unknown, there can be knowledge of their consequents. Since, then, things cannot be uncaused, and cause is not convertible with effect, and infinite regress is excluded, it remains that there is a first cause of all existing things, whence they severally proceed as branches from a root, some near to it and others more remote, For that there is not more than once such first principle has already been esablished, inasmuch as the subsistence of any manifold is posterior to the One (prop.5).