The first question of Ontology is "What is there?". That's easy. The answer is "Everything". The second question is the stinker. "Why is there anything?". That is where every other ontology that I know of fails. They tend to be arbitrary at this point and skip a step or weeble warble and obfusticate a lot.

Blavatsky took this head on and decided that the Theosophic Ontology had to start with a single axiom. Said axiom required to be Necessary, Generative, Self-Defining and Unique. Not bad for a single axiom.

Necessary because it must not be optional or arbitrary in any sense at all. It must not be a thing, action, relationship, abstract or concept requiring anything other than itself to define itself. Note that that leaves out any kind of quantum singularity, unmoved mover, immanence or deity as a starting point. Whatever it is it must be independent of any sort of observer or self observer effect and not be even eternal much less temporal.

Generative because if it isn't then it just all stops right there. It must be a tautology without being trivial.

Self-Defining because it must involve definition in order to define everything but it must not be defined by anything else.

Unique because it must be the only possible starting point in order to define everything that could be as well as everything that couldn't be.

Everything similar to it in any way must be it.