  The One is the absolutely simple first principle of all.  It is both ‘self-caused’ and the cause of being for everything else in the universe.  There are, according to Plotinus, various ways of showing the necessity of positing such a principle.  These are all rooted in the Pre-Socratic philosophical/scientific tradition.  A central axiom of that tradition was the connecting of explanation with reductionism or the derivation of the complex from the simple. That is, ultimate explanations of phenomena and of contingent entities can only rest in what itself requires no explanation.  If what is actually sought is the explanation for something that is in one way or another complex, what grounds the explanation will be simple relative to the observed complexity.  Thus, what grounds an explanation must be different from the sorts of things explained by it.  According to this line of reasoning, explanantia that are themselves complex, perhaps in some way different from the sort of complexity of the explananda, will be in need of other types of explanation.  In addition, a plethora of explanatory principles will themselves be in need of explanation.  Taken to its logical conclusion, the explanatory path must finally lead to that which is unique and absolutely uncomplex.