When something is said to be related to another it is as saying the two are referred to each other. There are many such relations and they apply to the things referring to each other. With brothers, they have a common parents and this is a real relation. If two things are relational they then have a real relation. With God there real relationships which are real proceedings. These real relations are of the same nature and so are by identity. But we cannot mistake the relations in God to be accidental because God has no accidentals. Any relations that exist of God are in fact in God, and have divine essence as well.
The real relations in God has real distinction as well, for anything with real relation indeed also has a real distinction. Two brothers are distinct but related. The separated persons of God, therefore, must be indeed distinct, but must also be in unity. This unity is one which unifies the essence, existence, and substance of a thing. Therefore they are distinct persons, but the same nature.
All of this considered we observe four real relations in God: Father, Son, and the Spirit which also proceeds (Q27). The relation is God to His Son, the Son to His Father, and both willing the procession of the Holy Spirit. Each of these is divine in nature for was God begets and generates, and also proceeds, are not separate from the one unifying nature, but are only distinct persons of the same nature.
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