There’s a website that answers this pressing question! It’s an interesting snapshot of philosophy and elite intellectual culture at the time. Among the guests was the logician Henry Sheffer, about whom we learn:

Henry Maurice Sheffer (1882–1964) was born in Odessa, Russia (now Ukraine) later becoming a naturalized citizen of the United States (1904). He earned his bachelor’s, master’s, and doctorate at Harvard University, studying under Josiah Royce. Sheffer’s self-confidence as a scholar can be seen in his claim of a major error by his thesis advisor, Royce, in an appendix to his dissertation. He was hired to join the Harvard philosophy faculty as an Instructor shortly after Royce’s death in 1916 and he stayed until his retirement in 1952. As it turns out, Whitehead might have played some small role in the effort to keep Sheffer at Harvard.

According to O’Connor and Robertson, “since Sheffer was Jewish, Harvard University insisted his salary should be provided by outside sources.” This claim finds corroboration in a 1927 letter to Whitehead from Felix Frankfurter (who was also Jewish and at the birthday symposium; see above). Regarding what Frankfurter calls the “the whole Sheffer business” he says “There seems a real ground for believing that about $150,000 can be raised for a research professorship in the general field of inquiry pursued by Sheffer for Harvard University.”  He explained that the only requirement of the donation was that “Sheffer be made the first incumbent.”[30]

In Lowe’s recounting of the reception of Whitehead’s first lecture at Harvard in 1924, Sheffer is said to have muttered as he left the room, “Pure Bergsonianism: Pure Bergsonianism!” As Lowe notes, at Harvard, “that was name-calling.”[31]

For his part, we know that Whitehead had a very high opinion of Sheffer. In a 1927 memo to an unknown recipient—likely written in support of Sheffer’s promotion to Assistant—Whitehead writes: “Symbolic Logic, as the general theory of structure, has been generalized to its utmost extent by Dr. Sheffer. He has not only enunciated the general principles, but achieved the far more difficult [task] of making substantial progress in constructing the groundwork of the science.”[32] He then goes on to note his and Russell’s reliance on Sheffer’s method in Principia Mathematica. Sheffer introduced what is now known as the Sheffer stroke in 1913, but it was its use in Principia that helped it to become well known. In Sheffer’s 1926 review of the second edition of Principia, Sheffer contends that Whitehead and Russell have “by a meticulous step-by-step analysis . . . arrived, in terms of formal logic alone, at the ‘front door’ of cardinal arithmetic.”[33]

In a surviving letter fragment to an unknown recipient several years later (likely 1929, given that Whitehead indicates he had been at Harvard for five years), Whitehead is even more glowing in his estimation, writing that “Sheffer is one of the most brilliant speculators and systematizers in this region of thought [applied mathematics].”

Several of the leading philosophical lights of the day in attendance are now mostly forgotten and little read (e.g., William Hocking, Paul Weiss, Ralph Barton Perry, Ralph Blake, William Montague, among others–all had excellent pedigrees, however!).