“Truth and reality in art begin at the point where the artist ceases to understand what he is doing and capable of doing …”
What Matisse is talking about here may be a general feature of creative processes. While they are going on, we don’t understand completely what we are doing.
In the creative process, the human being extends itself. You develop a new way of thinking, perceiving or acting. Before and while extending yourself, you cannot understand what is going on because the new way of thinking is outside the scope of the previously existing knowledge; the process leading there is not covered by the knowledge you started with. It is new exactly in that sense that it is outside of what can be derived and understood in the previously existing framework of knowledge.
So the process can only be understood in hindsight, after it has happened. Only afterwards we have a framework within which the new way of thinking and the process leading to it can be understood and from where it may even appear as a normal process. But that extended framework has been generated in the creative process and did not exist before.
So when you cease understanding completely what you are doing, you might be on the track to something new. Of course, such a jump might also lead nowhere (and most of them turn out to be false starts), but you can know that only afterwards.
(The picture, showing Henri Matisse, is from https://commons.wikimedia.org/wiki/File:Henri_Matisse,_1913,_photograph_by_Alvin_Langdon_Coburn.jpg).
 Mathematically, the framework of existing knowledge existing at any given time may be thought of as an algorithm or formal theory. Thought processes occurring within this framework can then be thought of as derivations inside a formal theory or computations performed by an algorithm. However, a formal theory or algorithm cannot describe or perform its own extension. What is computable by an algorithm or derivable inside a formal theory is so right from the beginning. It cannot become computable or derivable within the given formal framework. The process of “transmutation” into a new, extended formal framework can be described, in mathematical terms, as the execution of a “productive function”. This mechanism must exist outside the formal framework and be applied from the outside. If it is integrated into it, the resulting framework will be limited again and can, in turn, be extended by applying the productive mechanism from the outside. The process leading from the previous state of the framework of knowledge to the new, extended one, cannot be described in the previous framework, but may be describable in the extended one, becoming understandable in hindsight.