In one sense, a person is just a subject possession a rational nature. In another sense, however, 'person' is a forensic term, as Locke said it was: a person is that which is responsible before another, or to another, or for another, or to whom we are responsible. This sense, however, presupposes the other, and needs to do so because we don't call 'person' anything to which, or for which, we are responsible; rather, we only call rational agents 'persons'.
Nonetheless, the distinction is important, because the second sense, being forensic in nature, is much more vague, but is also the primary one that shows up in most of our thought about persons. A good example to show the importance of the distinction is Rostand's Cyrano de Bergerac. In Cyrano de Bergerac Roxane is misled into thinking Christian (who is brave and beautiful, but not so bright in matters of love) is brilliantly eloquent; this becomes at some point her primary basis for loving him. In actuality, all the brilliant eloquence she thinks belongs to Christian actually belongs to Cyrano (who is brave and brilliant, but grotesquely ugly). Now, until the very end, Roxane thinks she is in love with one and only one person; and, in a sense, she is (as she says toward the end, when both Christian and Cyrano are dead, she only ever loved one man, but lost him twice): forensically, there is only one person who forms the object of her love. The tragedy, of course, is that while this forensic sense of 'person' presupposes the ontological sense, it only does so vaguely, and this allows slippage and confusion. And in this case, the slippage and confusion means that Roxane acts as if Christian-as-she-loves-him were simply Christian, when he is really in some sense Christian and Cyrano both. Two subjects, treated as one forensic person. And the interesting thing is that this only becomes a problem because the narrative forces us to regard the separation between Christian and Cyrano (1) from their perspective, as two men in love with the same woman: Christian is forced to think that Roxane really loves Cyrano, because what she says she loves about Christian simply are all the things he has (as it were) only on loan from Cyrano; and since Roxane marries Christian, we see the tragedy of the separation from Cyrano's perspective as well; and (2) from Roxane's perspective, as she learns that she was misled for so long, and tries to sort out who she loved, and how. If there had never been anything to force anyone to consider the separateness of Christian and Cyrano, however (if, for instance, Cyrano didn't love Roxane, and if Roxane never learned that Christian's brilliance was borrowed from Cyrano), there would be no tragedy: the use of the forensic sense of 'person', which is a very practical use, would for all practical purposes be indistinguishable from the use of the ontological sense, even though there was still a small amount of theoretical slippage and confusion.
The relevance to Christian prayer (and Jewish prayer from a Christian perspective) and the Trinity should be clear.
