From Thomas Aquinas' Introduction to Aristotle's Posterior Analytics

" Reason not only can direct the actions of the lower parts, but it also directs its own actions, because it belongs to the understanding part to reflect upon itself. Indeed, the understanding understands itself, and in the same way reason can reason about itself. Therefore, if the arts of building and making, through which man can do such actions easily and in an orderly way, were discovered because reason reasoned about the actions of the hands, for the same reason there must be some art which directs the actions of reason itself. Through this, man proceeds in the actions of reason in an orderly manner, easily, and without error. "