Only supposed, a causeless cause is not possible, why HBP wrote about it in her teachings?
OK, let me explain. I describe it now in a theosophical way in regard to the spiritual hierachies. Not all of these are visible, and those parts that are visible, are visible only in the plane of matter of course. Hence we can also go the physical way (don't mix this up with modern astrophysics, which is often a hoax, see f.e. here and here or more elaborated in regard to postmodern, materialistic science here, with greetings from the Kali Yuga).
Anyway, let's start: A human being, the spiritual cause of it, is a planetary beeing (at least in the sense of it's monad, see f.e. SD:1:216c). Let's call the planet Earth. One could say, in this meaning, that the cause of this planet is the solarsystem, right? So the cause of our solarsystem -- which has, by the way, no name so far -- is our galaxy, called the Milky-Way. And the cause of our galaxie is..........oh... whait... we don't know / can see so far, what the cause of all the galaxies is. Even if we can speculate, we don't see or know such an entity, which consists, at least, of all galaxies we know.
Exactly here is obviously a border of todays knowledge. So let's call this "thing" that we don't know, this "Unknown Deity", this "Nothingness", which "Is (in existence) and is not (within our circle of understanding)", the Causeless Cause. Oh, sorry, let me correct this: I meant ""Causeless Cause"". Why? It is NOT to be meant literally.
No one would deny, that there is a cause of all our Galaxies. An no one would argue, that we already know everything, what's to know in this endless space. There is a cause, but we don't know it and call it, until we know better, the "Causeless Cause", because if we don't know the Cause, it's evident, that we cannot even speculate about the Cause if this Cause. It would make no sense.
So the galaxy, so to say, is the causeless effect. It has a cause, but we don't no it (thus far). And this unknown cause is called the "Causless Cause". There may be other opinions on this, but to assume, that there is an effect without a physical or metaphysical cause and/or to think, that there is an end of the Hireachy or of the Development or the Causes, it's not logical and therfore (politely expressed) highly problematic.
I don't want to destroy the Absolute and consequently the (Original) Teaching. You can call this the Absolute too, the One Reality etc. But on the other hand, we have to draw this "beginning" for a philosophical correct start of a Cosmogenesis (it is necessary, yes, see f.e. here) and if you want to go further with this topic, or deeper with it's understanding, you can divide this "thing" in the Unrevealed Logos and the Absolute. Maybe I discuss this in the next issue but for now...
What do you think?