Something that is only a concept in our minds could be greater by actually existing. And, of course, to draw attention to the sources of Thomas's philosophy is not to say that everything he holds philosophically can be parsed back into historical antecedents, or that he never disagrees with his sources, Aristotle in particular.
Herder, Its truth is manifestly obvious, and thus Aquinas employs it as an argumentative point of departure. Whatever is in motion, Aquinas states, is moved by something else.
But they are Socrates' activities as agent just as much as is the operation of intellect. So there exists a self-existent being. Thinking either is or is not merely a physical process and antecedent expectations do not settle the question, however much they influence the pursuit of that objective resolution.
Since Aquinas is dealing specifically with transcendentals like being and goodness, and since there is nothing outside the transcendentals, it follows that there is nothing outside the genus which could be a cause condition 2.