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.
*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 relatively minuscule - merely a lovely pebble or shell compared with the whole ocean.)