What are the key differences between (classical) pragmatism and conventionalism? I'm reasonably familiar with the first, and have just become aware of the second via a reference to Henri Poincaré's La Science et l'Hypothèse. From my current perspective the two doctrines appear at least broadly similar.

Here is a quote from Charles Sander Peirce on pragmatism:

Consider what effects, which might conceivably have practical bearings, we conceive the object of our conception to have. Then, our conception of those effects is the whole of our conception of the object.

And here is Wikipedia on conventionalism:

Conventionalism is the philosophical attitude that fundamental principles of a certain kind are grounded on (explicit or implicit) agreements in society, rather than on external reality.

Both descriptions refer to the practice of human discourse ("practical bearings", "agreements in society") rather than any metaphysical objects, hence the (perhaps) considerable similarity. There are some obvious differences around histories (U.S. vs. France, philosophy of religion in the case of William James vs. natural sciences, individual vs. societal dimension), but I'm mainly interested in (other) epistemological key differences here. I have already checked e.g. SEP, which does seem to address the matter either.


What are the key differences between (classical) pragmatism and conventionalism?

Conventionalism doesn't refer, at least in itself, to practical bearings. It is the view according to which there is no valid certain knowledge, no informational content about the world but only inter-subjective conventions. So it is essentially a claim regarding the nature of what we call knowledge; it is an epistemological stance.

Pragmatism, on the other hand, is an all embracing approach for philosophical conduct. To relate to Conventionalism, one might inquire into the epistemological conclusions of Pragmatism:

  • As a theory of justification, it is the thesis that statements are justified only as a function of how consistent they are with the other statements in the system. Not true for Conventionalism, as the latter isn't dependent upon the consistency of the conventional content; inconsistent as it may be, a convention could still hold firm.
  • As a theory of truth, it is the thesis that the property of Truth of a statement is essentially the property of being practical (or useful) to hold. Again, not true for Conventionalism as it isn't necessary for convention to be practical; it only has to be mutually accepted, be it practical or not.

Both descriptions refer to the practice of human discourse ("practical bearings", "agreements in society")

Good point :)

  • @Drux I've edited the answer, thinking it might now be more to the point. Feedback is appreciated. – SkepticalEmpiricist Sep 7 '13 at 22:45

I posted this self-same answer as a second response to your query about the relationship of sophism and postmodernism, noting that the relationship between that question and the instant is worthy of comment. On reflection, I thought my answer belongs here, rather than as a response to your sophism/postmodernism query. So here it is, verbatim (be patient, I am new to this site). Very broadly speaking pragmatists vis truth hold that what is true is NOT what corresponds to "reality", but what works (eg James: The true is the expedient in the way of our thinking on the whole and in the long run. Caveat: Like Wittgenstein's "meaning = use" ["the meaning of a word is its use in the language"], these epigrams are simply place holders allowing us pithy communication, but they fail miserably to capture the true complexity of ideas they [seem] to express). Extreme Conventionalists (eg: Mach) might argue that scientific laws are not discoveries of how the world is/works but "in fact" conventions that reflect decisions to adopt one of various possible descriptions (again, shades of Rorty's redescriptions and possibly even Wittgenstein's normativity/ language games). Note the possible relation/ similarity: Why adopt one description rather than another? Not because it "corresponds" with "reality" (the druthers of a realist] but because it is expedient. Because it works. Likely the most influential intellectual move associated with the Greek sophists (as with with Socrates) is to highlight the difference between nature (physis) and CONVENTION (nomos), which gave rise to the aforementioned skepticism which they share with the postmodernists, and their infamous/ disreputable teachings of how to argue [win arguments] rhetorically, that is, independently of the "merits of the case", or "the fact of the matter" [ie, sophistry]. However, note Protagoras’ [arguably the most famous of sophists] skeptical credo "Man is the measure of all things", with the postmodern implication that there is no [unique/indispensable] “fact of the matter” or “merits of the case” since physis and nomos are inextricably intertwined (Until Kant came along to disentangle them by addressing what was by then Humean skepticism).

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