I don't assume Quine to be any different from anyone else in recognizing his influences.

It is because I have no particular confidence in anyone recognizing their own influences that I turn to timing to help me answer the question of independent creation.

1) If a person is the first person to give public expression to an idea, then the chance is relatively high that he is the originator of the idea. It's not completely certain, but it's relatively high.

2) In contrast, if a person is not the first person to give public expression to an idea but is, say, the 437th person to do so, the first having done so fifty years before, then chances are relatively high that he picked up the idea from somewhere and didn't remember picking it up. The fact that nobody expressed the idea before fifty years earlier suggests that the idea is pretty hard to come up with independently, because had it been easy, people would have been coming up with it all through history.

3) Finally, if a person is not the first person to give public expression to an idea but people have been giving public expression to the idea for as long as we have records, then the chance is relatively high once again that he independently rediscovered the idea, since it seems to be the sort of idea that is relatively easy to rediscover independently.