Rifkin’s logic can be expressed in a syllogism. The major premise is that Bach provided his singers with written parts meant for use by a single performer only. The minor premise is that in all but a few of Bach’s surviving choral works only one part was ever written out for each of the voice ranges. If both premises are accepted, the conclusion is obvious: Bach used a solo quartet to sing the choruses.

The minor premise, that Bach usually prepared only one part for each range, is the standard view among Bach scholars. The major premise, however, is revolutionary. Scholars have long believed that Bach’s singers shared their parts, typically in groups of three–a principal singer, who sang everything in the part from beginning to end, and two supporting “ripienists,” who joined in only for the choruses and chorales. But according to Rifkin, there is no real evidence for the ripienists sharing the parts, either in documents or in the parts themselves.

All agree that a typical Bach voice part included every movement in the work, both solo and choral, for its particular range. If the ripienists read from such a part, they would have to know when to sing and when to stay silent (the principal singer, all agree, would sing throughout). Yet the parts consistently fail to provide reliable cues for when ripieno singers should enter and exit - even in complex situations that seem to demand such cues. Since Bach did write other performance cues into these parts regarding articulation and tempo, the explanation can't be that he didn't have time to notate the ripieno entries, or that he typically covered such matters only in rehearsal. Rifkin adds that a substantial number of parts show positive signs of having been intended for use by one singer only. Several bear character designations (“Anima,” “Jesus,” “Phoebus”), and some indicate solo singers in the list of forces on the wrapper in which Bach kept the parts (the vocal scoring for BWV 56 reads “S. A. T. &Basso Con[certato]”). Even those few works for which Bach did provide special ripieno parts, says Rifkin, support the major premise that single voices read from the principal parts. --from the book Inside Early Music