Primary Thinking (or Pure Thinking, as Rudolf Steiner generally terms it*) has no limits to potential knowledge, and is fully conscious, self-aware.
This means that because we are fully conscious of our thinking (and active, not passive, in the free agency of this thinking) - we can know (for sure) when we are 'doing it'; and of course when we are not; and we can therefore chart our own progress.
On the other hand; given the unbounded power and scope of primary thinking (that is in the realm of truth, beauty and virtue) - we also know with certainty how very partial, embryonic, our own achievement actually is.
And by Primary Thinking we attain to what Barfield terms 'final' participation - final because it is true, it has no limits and is conscious; in other words it is thinking as a god (Son or Daughter of God) thinks; and participating, as a god (Son or Daughter of God) participates.
Once Primary Thinking is attained, fully and continuously, there is no further to go in terms of mode of consciousness (although, of course, there is an unbounded amount of knowledge content which might potentially be known, and can only be learned by serial addition...)
*This can be discovered in his magnum opus 'The Philosophy of Freedom'; which is very helpfully expounded and interpreted in 'Rudolf Steiner on his book The Philosophy of Freedom', edited by Otto Palmer.
(Since primary thinking is the creativity of a genius, the combination can be illustrated by Isaac Newton - who knew that he was one of the greatest of mathematicians and physicists; yet also knew that his discoveries, while significant, were - relative to what could be known - minuscule; a shell on the beach compared with the ocean of truth.)