What follows is my (very rough) translation of the first article of Aquinas's question on the Father. The Latin is here, the Dominican Fathers translation is here.
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We proceed to the first [article] in this way.
[1] It seems that the Father cannot be said to be the Principle of the Son or of the Holy Spirit. For principle and cause are the same, according to the Philosopher. But we do not say that the Father is the Cause of the Son. Therefore one ought not say that He is His Principle.
[2] Further, principle [principium] is said with respect to what is of the principle [principiati]. If therefore the Father is the Principle of the Son, it follows that the Son is of the Principle, and consequently is created. Which seems to be erroneous.
[3] Further, the word 'principle' is taken from priority. But in the divine there is no prior or posterior, as Athanasius says. Therefore in the divine we should not use the word 'principle'.
But to the contrary is what Augustine says in De Trin. IV: The Father is the Principle of the whole deity.
I respond that it must be said that this word 'principle' signifies nothing other than that from which something goes forth [procedit], for everything that goes forth from another in whatever way, we call a 'principle'; and vice versa. Since the Father is one from which another goes forth, therefore, it follows that the Father is Principle.
Therefore to the first it must be said that, in divine things, Greeks use indifferently the word 'cause' and the word 'principle', but Latin teachers do not use the word 'cause', but only the word 'principle'. The reason for this is that principle is more general [communius] than cause, as cause is more general than element, for the first term of a thing, or the first part of a thing, is called 'principle', not 'cause'. But to the extent some name is more general, it is more appropriate to apply it in divine things, as is said above, because names, to the extent they are more specific [magis specialia], are more determinate in a way appropriate to creatures. Thus this word 'cause' seems to imply diversity of substance, and dependence of one on another; which the word 'principle' does not imply. Therefore in all kinds of cause, there is always discovered a distance (according to some perfection or power) between the cause and what is caused. But the word 'principle' is used in even in cases that do not have a difference of this kind, but only a difference of order, such as when we say a point is the principle of a line, or also when we say the first part of a line is the principle of the line.
To the second it must be said that with the Greeks one finds it said of the Son or the Holy Spirit that they they are of the principle [principientur]. But this is not the usage of our teachers. For although we attribute to the Father something of originality [aliquid auctoritatis] through the notion of principle, nonetheless we do not attribute to the Son or the Holy Spirit any subjection or inferiority of any kind, so as to avoid all occasion for error. Accordingly, Hilary says (De Trin. IX) By originality of the giver the Father is greater; but the Son is not less to whom one being is given.
To the third it must be said that, although this word 'principle' seems to be taken from priority, to the extent [we consider] that from which its signification is imposed, nonetheless it does not signify priority, but origin. For what a word signifies, and that from which the word is imposed, are not the same, as is said above.
