The reference is to Aristotle, Physics III, 6. The whole chapter is worth reading, but these words in particular: "The infinite, as such, is unknowable. . . . . We must not take the infinite to be any one definite reality, as a man, or a house, but in the sense in which we speak of 'the day' and 'the performance,' entities of which is predicated no substantial reality, but a reality that consists in perpetually coming to be and ceasing to be; a reality which, though limited, is continually other and other. For the infinite is not that, beyond which is nothing, but beyond which there is always something." Aristotle then does not admit the possibility of the actual infinite, full and complete, but only of the series running on without stopping, and never reaching a final term, which is called potential infinity.

Of God and His Creatures: 1.69