De Trinitate

"But whatever hesitations there may have been generations ago, 'person' is now almost universally accepted as the word of choice to describe the quality of being in common to all three members of the Trinity and it seems unlikely that it will be dislodged from its exalted position anytime soon. Concomitant with this has been the apparent eclipse of the basically philosophical concept of 'substance.' This has been a long time coming, and it may not be definitive, but for the moment at least, it seems possible to say that the notion that God is a 'substance' somehow distinct from his three persons has been rejected, mainly on the grounds that the unity of God cannot be depersonalized, nor can there be a fourth 'thing' in him that can be conceptualized as such."

---Gerald Bray, The Trinity: Where Do We Go From Here?, in _Always Reforming: Explorations in Systematic Theology_, IVP, 2006, p.23

Also see this post by James Anderson, as well as some corroboration of some of Anderson's comments by Bray (ibid, pp. 30-31).