" my point was that the only reason for a society to grant patents is to provide a viable alternative to the former system (closely held trade secrets) without the risk of the secret dying with the inventor?"
I guess my question would be WHY you see ONLY this reason, and refuse to acknowledge the others. I mentioned at least one of them. But you have rejected it without any real argument or refutation, and simply repeated your original statement again.
The fact that inventions were created before the motivation of patents existed, is not evidence that patents do not create motivation. The real question, which you have refused to even acknowledge so far, is: which is BETTER? A system with no patents, or a system with patents.
Actually, you're changing the argument here. This part of the discussion was about why patent laws were enacted in the first place (was it to motivate people to invent, or to motivate them to disclose the details of their invention?). It was never about whether patents do or don't motivate people to invent thing, only about whether the supposition that they do was behind the creation of the patent system.
You argued that this was "obvious" from the constitution by imposing a modern perspective--shoe horning a Randian perspective on a document written a century and a half before that view gained currency--and a bit of selective reading. I countered that given the prevailing circumstance (e.g. trade secrets as a prevalent practice) and the clear written statement (e.g. the law itself, which I cited above) a much more probable explanation was that the intent was to motivate disclosure of existing inventions rather than (as you would have it) invention per se.
This may seem odd to modern sensibilities, in a world where "the profit motive" is taken for granted (and condoned) and we have more information at our fingertips than we could possibly digest, a world where cases such as starlite (which may well be a fraud in any event) seem like musty relics of pre-Victorian era, but I think it's safe to say the founders of our nation would have had as hard a time seeing things from our perspective as we have seeing it from theirs.
Likewise, as for your question about my phrase "the only reason for a society to grant patents" I think you are confusing motivations of the two parties (society and the inventor). There are many things that might motivate an inventor (dreams of wealth, fame, glory, desire to scratch an itch, prove a point, discomfit a rival, etc.) but society as a whole is largely indifferent to these. If we are to be strictly randian (as seems to be the tenor here, at least in so far as the constraints of historical accuracy permit) the only thing that works as a societal motivation is something that benefits people in general, imposing a cost on (in an ideal case at least) no one but the inventor. The most salient of the possibly candidates is clearly disclosure--we all gain information, and the inventor is out one secret.
I will, though, admit that "only" was too strong and there are indeed other (far less plausible) candidates. Perhaps we all love a Horatio Alger tale enough to want to foster them, or can't help but indulge our schadenfreude habit when a mustachio twirling industry is turned on its head by a plucky upstart. But I haven't been able to turn up any contemporaneous support for these theories.
By your argument, I could claim that firearms are not effective for hunting because animals were killed long before firearms came along. I don't buy it. It's not black and white, it's a matter of degree.
Again, I believe you are getting yourself tangled. You started this line of discussion by making the contrary black and white claim:
You: The idea (which history supports) being that when you don't allow people to profit from their own efforts, things don't get invented.
Me: That would make sense if there was a shred of evidence that people only invent things because they hope to patent them.
You: Well then, it makes sense, because we have far more than a shred. We have at least 300 years of historical evidence, continuing into modern times.
...and I objected, pointing out that history very clearly show that things were invented before patents, and that patents are not, as you seemed to be arguing, the only (or even the best) reason or people to invent things.