MacKenna-Plotinus: principle of identity

If the Alienism is to be understood as meaning only that Matter is differentiated, then it is different not by itself [since it is certainly not an absolute] but by this Difference, just as all identical objects are so by virtue of Identicalness [the Absolute principle of Identity]. Enneads   II,4,

It may be suggested that its existence takes substantial form only by its being resident among outside things: but, at this, it is itself no longer simplex nor could any coherence of manifolds occur. On the one hand things could take substantial existence only if they were in their own virtue simplex. On the other hand, failing a simplex, the aggregate of multiples is itself impossible: for the simplex individual thing could not exist if there were no simplex unity independent of the individual, [a principle of identity] and, not existing, much less could it enter into composition with any other such: it becomes impossible then for the compound universe, the aggregate of all, to exist; it would be the coming together of things that are not, things not merely lacking an identity of their own but utterly non-existent. Enneads V,6,

Yet again: In any dual object there is the unity [the principle of identity] side by side with the rest of the thing; an associated member cannot be the unity of the two and there must be a self-standing unity [within the duality] before this unity of members can exist: by the same reasoning there must be also the supreme unity entering into no association whatever, something which is unity-simplex by its very being, utterly devoid of all that belongs to the thing capable of association. Enneads V,6,