Whoever thinks algebra is a trick in obtaining unknowns has thought it in vain. No attention should be paid to the fact that algebra and geometry are different in appearance. Algebras (jabbre and maqabeleh) are geometric facts which are proved by propositions five and six of Book two of Elements.
As we know, there are known knowns; there are things we know we know. We also know, there are known unknowns, that is to say, we know there are some things we do not know. But there are also unknown unknowns, the ones we do not know we do not know.