Note: the following information was provided to me by Jessica Wilson and David Balcarras, also referencing Benj Hellie.
David Lewis and Robert Stalnaker are commonly credited with the "Lewis-Stalnaker analysis" of counterfactual conditional in terms of nearby possible worlds--an analysis that has become so dominant in analytic philosophy that it is typically just assumed in professional articles and discussion. But, should they be the only ones credited for the theory?
Stalnaker initially published his theory of counterfactuals in a (1968) article, and Lewis his theory in a (1970) article in the journal Inquiry critiquing and expanding upon Stalnaker's initial proposal. However, the very same year (1970), Timothy Sprigge published a book. Facts, Words, and Beliefs, defending an analysis of counterfactuals identical in almost every way to Lewis' now-famous analysis.
Now, although there is (to my knowledge) no evidence of malfeasance on Stalnaker's or Lewis' parts, there is evidence that Lewis learned of Sprigge's contribution much later (in 1993 two decades later), as shown in the following letter to Sprigge (quoted verbatim from Sprigge, Timothy L. S. (2006) "My Philosophy and Some Defence of It" In Consciousness, Reality and Value: Essays in Honour of T. L. S. Sprigge. Ed. by Pierfrancesco Basile and Leemon B. McHenry. Ontos Verlag. pp. 299--321):
Dear Professor Sprigge,
Thank you very much for the note and photocopy you gave me when I was in Edinburgh last month. Since returning to Princeton, I've read Facts ... XIl.5 and Vindication ... 2.5. Sure enough-your 1970 analysis of counterfactuals is very like Stalnaker's and essentially identical to mine. (The only difference I spot concerns the case of an impossible antecedent: your counterfactual goes false, mine and Stalnaker's go true. Very much a side-issue.) I wish I'd known about your analysis and given you due credit years ago, but better late than never!
As for chronology, it's all very close. It was in May 1968 that I wrote to Stalnaker saying, "You and I have proposed very similar theories of counterfactuals". (I'd just heard of his work from Richmond Thomason.) At that point he had a final version of his soon-to-bepublished "A Theory of Conditionals"; I had seminar notes; and if my experience with production of books is any guide, you must already have had a manuscript of Facts . .. in pretty much final form. Stalnaker' s paper appeared later in 1968; my lheory as told by Howard Sobel (with my permission and with full acknowledgement) appeared in 1970 in Inquiry, as an appendix to Sobel's paper on utilitarianism; your book and Stalnaker and Thomason's joint paper appeared that same year; and my own first paper on counterfactuals appeared in Theoria in 1971. Paul Benacerraf beat us all by many years. But alas, he let Carnap persuade him that because of the vagueness of similarity, the idea wasn't worth pursuing!
If I believed in complex structural universals, one of which is the total nature of the actual world, and the rest of which could have been, then I might be willing lo join you and others in identifying these structural universals with (at least some ot) the possible worlds. I resist not the identification, but rather the structural universals themselves. I have trouble making sense of the relation between a structural universal and the simpler universals which are its constituents. I'm sending you an offprint about this from the Australasian JP, 1986.
Sincerely (Signed David Lewis)"
Unfortunately, although Lewis writes in this letter that crediting Sprigge with co-developing the theory is "better late than never", as of the present-day it doesn't seem better late than never. Sprigge is almost never mentioned in connection with the theory of counterfactuals, and indeed, is not even referenced in the Stanford Encylopedia of Philosophy entry crediting the theory primarily to Lewis (as well as, secondarily, to Stalnaker).
Is it time for philosophers to stop referring to the "Lewis-Stalnaker theory" of counterfactuals, and instead start calling it the Stalnaker-Lewis-Sprigge theory?