Mr Jones comments in the Cross thread:
The rest of what you say is fine. Here is the problem: . . . What your saying is that the Son inherits the principle property of causing a divine person because "what he has is only the Father's, i.e. the substance of the Father"? So if the Son inherits the principle property of causing a person, so should the Spirit since He inherits the same substance. By your gloss Principle and to be Father are not co-extensive, since the Son shares it. So it is not an exclusive personal feature. On my view, principle is the exclusive personal feature of the Father.
I agree that some clarification is needed here. At the same time Mr Jones' objection seems ambiguous at best. I would not admit that there is any "principle property of causing a divine person"--first because this again seems to be equivocating on "principle" as it's being used here; second because, as EP have themselves rightly pointed out in past discussions, "causing a divine person" is not a personal property in divinis. It is a fact that both the Son and the Spirit are "caused", but there is no real generic property of which generation and spiration are species.
The real problem, namely why the Son does not inherit generation and the Spirit spiration, since they both inherit the divine substance which is the principle of all action, needs more elaboration. For my purposes I will use Scotus' Reportatio I-A, which has the merit of being published alongside an English translation by Allan Wolter and Oleg Bychkov, sparing me the trouble of making my own translations. I make some slight emendations, however, in the snippets which follow.
I turn then to Dist. 7 Q.1, "Is the principle of producing in the divine a relation or the essence, or is [it] something absolute or relative?" This is a long question, but the money quote is this:
the divine essence is the formal principle of producing some person--moreover, sufficiently without any determination--but it cannot proceed to function unless the personal property concurs. And if the formal principle of producing something is understood in this way . . . I concede that the relation concurs with the essence to produce the Son, not to determine the essence which is determined of itself, but in order that the latter may come to be in proximate potency for acting, in which i can only be insofar as it is an individual subject and person.
I.e., the essence is the entire sufficient cause for what is generated being God. But in order for generation to take place the personal property of the Father--generation--has to "concur" with the essence. It is not the essence taken all by itself that generates, but the essence concurring with Fatherhood, i.e. the essence precisely as existing in the Father qua Father. Therefore when Mr Jones says "On my view, principle is the exclusive personal feature of the Father," I say we must distinguish between the principium quod and the principium quo.
The Son, then, does not generate, and for the same reason the Spirit does not spirate. Here is Scotus in Dist.7 Q.2 on why it is impossible for the Son to generate:
If the Son had the potency to generate, either he would generate by the same generation as the Father does, or by another. Not by the same; for if he did, the Son would generate himself, just as the Father generates him. Not by another generation, because there is no more than one production of a given sort in God because each is of itself just this, as was proved above in distinction 2, and also each production is suited precisely to its productive principle; therefore in no way does the Son have the potency of generating.
There can only be one generation and one spiration in God. If they were two generations, there would have to be something to distinguish them. But what would this be? There is no "principle of individuation" in God besides Himself. The Son cannot generate because, if he did, his generation would be identical with the generation of the Father, and thus what the Son would generate would be the Son, and thus the Son would generate Himself, which is contradictory and absurd. It is not contradictory and absurd for the Son to spirate, because the Son is not the Spirit. The Son receives his ability to spirate from the Father along with everything else pertaining to His Sonship. But the spiration of the Father and the Son is one spiration, not two, for there can only be one spiration in God. It is contradictory, however, for the Spirit to spirate, because what is spirated in God is the Spirit, and thus the Spirit would spirate Himself.
Also relevant is this bit later in Dist. 7 Q.2:
It must be said that this is not a precise expression: 'the essence is the principle of generation.' Indeed it is a truncated version unless it is specified 'the essence is the principle of generation for this one, namely the Father'; therefore it does not follow that there will be a potency to generate in the Son, indeed that is a fallacy of accident. For the essence is in the Son under such an aspect, under which the minor extreme, the potency to generate, is repugnant to him, as has been shown.
For divine generation both divinity and Fatherhood are required; in one sense divinity is the principle, since it is God which is generated, and in another Fatherhood is the principle, because it is the Father Who generates by His divinity; the Son does not have generating divinity but generated divinity. Similarly with the Spirit.
One final quote, from Dist. 12 Q.3:
This action of spiration can be considered in three ways: either in itself or towards another, or as it is in supposits acting. In the first two ways there is uniformity, just as if it were of one supposit. But in the third way, this action would not be uniformly from these supposits. For the Father has nothing that has been born, and whatever the Son has was received through generation. In this way, therefore, they would have the spiration action according to a certain order, and by reason of this a certain diversity could be asserted. And in this way one should understand the authorities; for I don't understand them in any other way.
There is only one spiration in God in itself. There is only one Spirit spirated. But there are two persons spirating, and the way they spirate is different, for the Father is the "principle" of spiration in this sense, that he spirates in virtue of his fontal plenitude, his being the source of everything in the Godhead, whereas the Son spirates in virtue of receiving everything He has from the Father. The Father spirates from Himself alone, then, whereas the Son spirates only through the Father and with the same spiration as the Father.
A very great deal more could be said, but perhaps this post is long enough. On final note. Mr Jones says "as far as explanation goes I don't find anything proffered by Bonaventure and Scotus that wasn't already covered by Alcuin and Ratramnus." I admit I don't know how to take this. Do Alcuin and Ratramnus really say what Scotus says in this post? If so I would be astonished. The increase in theologians' explanatory power from the beginning to the end of the thirteenth century alone is incredible, and also obvious to anyone to anyone who reads both early and late scholastics.