If you were to ask me, "What is a Proper?" I would answer, "It's an antiphon--and some verses of a Psalm if you have time."
Two things have recently made me wonder if I'm not looking at the Propers from a 180 degree wrong angle. Perhaps I have it backwards, and the introit is A Psalm, which alternates with an antiphon that shows the Psalm in greater light.
The first thing that caught my attention in this matter was the plenary address given at last year's Colloquium by Fr. Guy Nicholls of the Birmingham Oratory. Fr. Nicholls convincingly demonstrated that the Introit Psalms in Ordinary Time are not chosen according to the readings of the day or according to any other external device, but run sequentially through the Book of Psalms from beginning to end. In effect, the Introit Psalms of the year are a Psalter. If this is true--if we are meant to be singing the Book of Psalms throughout the year as monks sing them through the week--then the Psalms are much less incidental to the Propers than I had thought.
The second indication that I was taking the Psalms less seriously than they deserved, and perhaps taking the Antiphons more seriously than warranted, was this interesting definition of "Introit" from the old, online Catholic Encyclopedia:
The Introit (Introitus) of the Mass is the fragment of a psalm with its antiphon sung while the celebrant and ministers enter the church and approach the altar. In all Western rites the Mass began with such a processional psalm since the earliest times of which we have any record.I'm sure most of us have had the experience of needing the briefest possible proper, and if you are like me, you would have chosen to sing the antiphon and left the Psalm for another day when there was more time. However, I'm not convinced that this is the right way to go.
The article in the Catholic Encyclopedia, as I read it, seems to suggest that the Introit started as a Psalm only, and that antiphons were added later. Over time the antiphons came to be the defining aspect of the Introit, perhaps in part because the antiphon alone needed to be printed, in order to provide the melody. The Psalm would be sung to a Psalm tone, and by the cantor, who would not need it to be printed in the book. Eventually the Psalm was shortened to one verse with its Gloria Patri unless further verses were needed for reasons of time.
I wonder whether this is a unique scholarly view, or whether it might be safe to say that the Psalm is at least as essential to the singing of the responsorial Propers as is the singing of the through-composed antiphon.