Full Idea
If we can say 'there exists a p such that John believes p and Barbara believes p', logical forms such as this are cited as evidence for our ontological commitment to propositions.
Gist of Idea
If two people believe the same proposition, this implies the existence of propositions
Source
Alex Orenstein (W.V. Quine [2002], Ch.7)
Book Reference
Orenstein,Alex: 'W.V. Quine' [Princeton 2002], p.171
A Reaction
Opponents of propositions (such as Quine) will, of course, attempt to revise the logical form to eliminate the quantification over propositions. See Orenstein's outline on p.171.