  Plotinus’ writings were edited by Porphyry (there was perhaps another edition by Plotinus’ physician, Eustochius, though all traces of it are lost).  It is to Porphyry that we owe the somewhat artificial division of the writings into six groups of nine (hence the name Enneads from the Greek word for ‘nine’).  In fact, there are somewhat fewer than 54 (Porphyry artificially divided some of them into separately numbered ‘treatises’), and the actual number of these is of no significance.  The arrangement of the treatises is also owing to Porphyry and does evince an ordering principle.  Ennead I contains, roughly, ethical discussions; Enneads II–III contain discussions of natural philosophy and cosmology (though III 4, 5, 7, 8 do not fit into this rubric so easily); Ennead IV is devoted to matters of psychology; Ennead V, to epistemological matters, especially the intellect; and Ennead VI, to numbers, being in general, and the One above intellect, the first principle of all.  It is to be emphasized that the ordering is Porphyry’s.  The actual chronological ordering, which Porphyry also provides for us, does not correspond at all to the ordering in the edition.  For example, Ennead I 1 is the 53rd treatise chronologically, one of the last things Plotinus wrote.