Goethe said that in science, and presumably elsewhere, there comes a point where one must stop asking questions, stop analysing phenomena into smaller and smaller units and causal interactions - and this point he called the urphaenomenon - which translates as 'original phenomenon'.
One must stop questioning because the phenomenon is indeed 'original' that is it is an origin, which is also to say it is one of the primary creators of reality.
Ultimately, this might be understood as one of the 'origins' of creation - one of the phenomena that existed before divine creation - from which the divinity created; or else (by a different metaphysics) as one of the primary 'units' that were created by the divinity.
So reality began with urpheanomena; and what we perceive is a consequence of interaction between, addition of, transformation of (etc) the urphaenomena. They are the deepest attainable level of reality.
If we try to (pretend to) dissect the urphaenomena, or try to explain what causes them to be as they are - then we err; we get it wrong. Precisely because the urphaenomena are what make explanation possible, they cannot themselves be explained.
Instead of being-explained, the urphaenomena need to be understood. And we potentially understand them by an act of intuition.
How? Because by real, primary, intuitive thinking we may know directly. That is we may know by thinking ourselves that which is real - the tought is identical with the reality. We can - indeed - only know the urphaenomena directly (and not other kinds of phenomena), because urphaenomena are the only real and universal phenomena; we cannot know other kinds of things in this direct way.
Goethe was mostly concerned to define urphaenomena in relation to biology, to emphasise the livingness of living things; the way in which biology was based on a process of development, metamorphosis, transformation - and could not be understood in terms of physical structures (like DNA or brains) or physical processes (like diffusion or neurotransmission). When we do (try to do) this, we simply stop doing biology by treating the living as if it was not living - destroying the very basis of biology. Goethe also asserted that colour was ultimately an aspect of biology - and thus an urphaenomenon that cannot legitimately be redescribed in terms of combinations of wavelengths.
My opinion is that Goethe made a vitally important point - but my own understanding is that all of reality is living (or a part-of some-thing living) so there is no ultimate difference between physics-chemistry and biology - except that the difference is in the opposite direction than usually asserted (ie. all physics is ultimately biology, and indeed all biology is ultimately thinking - and all thinking is ultimately not-biological!)
I would say, more simply than Goethe, that the urphaenomena are Beings.
When we get to the level of disussing Beings we are at the level where free will or agency applies - in other worlds, the level where 'selves' are uncaused causes. We cannot understand a Self by looking inside it, nor by analysing it, nor by breaking into components - because this would be to break the Self, so that it was not being regarded as a Self. The one thing that cannot be done to a Self - qua Self - is to explain its 'inner workings'. Whatever we might suppose we have found; if is really is a Self, then we cannot have found its inner workings.
What we can do, perhaps, is to understand the process by direct knowing, by identification, which means to 'participate' in the process itself - which is at the level of ultimate reality, which is (a kind of) Thinking.
This is the way we (sometimes, briefly) understand or 'know' another person. And it entails loving that person; because love is the absolute necessity for direct knowing. Only by loving the phenomenon can a scientist know a phenomenon - thus real science is not a methodology but a product of sustained and truthful mtoivation to know reality.
In sum, we can legitimately keep asking questions as to what-causes-what, until we get back to the origins of causes; until we get back to uncaused causes; at which point we must stop, because there is in reality no observable cause of the uncaused; because we have reached the point at which this particular chain of causes originates.
