In earlier times...

Thinking of thinking is not a modern invention. It has already been described in Greek philosophy by Aristotle and in scholasticism by Thomas Aquinas.

Aristotle (Metaphysica)

“And thinking in itself deals with that which is best in itself, and that which is thinking in the fullest sense with that which is best in the fullest sense. The thinking spirit thinks itself because it shares the nature of the object of thought; for it becomes an object of thought in coming into contact with the thought, so that the thinking spirit and the object of thought are the same. For that which is capable of receiving the object of thought, i.e. the essence, is the thinking spirit. And he realises it by making it his own. Therefore the possession rather than the receptivity is the divine element which the thinking spirit seems to contain, and the act of contemplation is what is most pleasant and best. If, then, God is always in that good state in which we sometimes are, this compels our wonder; and if in a better this compels it yet more. And life also is in God; for the realisation of the thinking spirit is life, and God is that actuality; and God’s selfdependent actuality is life most good and eternal. We say therefore that God is a living being, eternal, most good, so that life and duration continuous and eternal being belong to God; for this is God.”

Thomas Aquinas (Commentary on the Gospel of St John)

“Secondly, from what has been said, we are able to understand that a word is always something that proceeds from the spirit in a living intellect existing in act; and furthermore, that a word is always a notion (ratio) and image of the thing understood. So if the thinker and the thought are the same, then the word is a notion and an image of the spirit from which it proceeds. On the other hand, if the thinker is other than the thought, then the word is not an image and notion of the thinker but of the thing understood, as the mental representation which one has of a stone is an image of only the stone. But when the spirit thinks itself, its word is an image and notion of the spirit. And so Augustine (On the Trinity IX, 5) sees an image of the Trinity in the Soul insofar as the mind thinks itself, but not insofar as it thinks other things. It is clear then that it is necessary to presume that the word is in all spiritual beings, for the understanding of thinking shows us that the spirit forms something by thinking: what is formed is called Word. And so we must presume the word in all thinking beings.”

What is new is that thinking of thinking can be practiced in our times fully aware and with free will.
 

Portrait of Aristotle, roman copy, created after the original of Lysippus, Paris, Louvre

 
Thomas Aquinas, Doctor AngelicusIn earlier times... by Mieke Mosmuller