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"Extraordinary claims require extraordinary proof." This is the mantra of skeptical thinkers and it provides useful for evaluating claims that fall outside the mainstream of science. This is part of the network of corroborating evidence that we demand as we review research claims in medical journal articles.
One aspect of a claim that makes it extraordinary is that there is no plausible mechanism that would explain how the therapy works. Therapies without such a mechanism would be subjected to a higher standard of proof. Don't reject a therapy automatically, though, just because no known mechanism exists. Many successful medical interventions were adopted before a mechanism was discovered that explained how and why that intervention worked.
Another problem is that not everyone agrees on when a plausible mechanism exists. Proponents of homeopathy argue that their approach works because water has "memory" that retains the effect of medicines that otherwise would be diluted out. They point to a series of experiments published in Nature magazine.
These experiments represent proof of mechanism, according to proponents of homeopathy. Critics, however, argued that this was an aberrant finding, or perhaps even fraud.
The editors of Nature acknowledged this unusual finding and then demanded a supervised replication of the findings. This is just common sense, according to the critics of homeopathy. The French scientists, however, cried foul. No other results reported in Nature ever needed supervised replication.
The supervised replication failed, of course, but then the proponents of homeopathy claimed that the replication was flawed and not the original finding.
So does a proven mechanism for homeopathy exist? Well, it depends who you talk to. Here are a few of the viewpoints I found using a simple Google search on the names of the authors of the Nature publication.
One of the sites has a quote used by Jacques Benveniste, one of the scientists in the Nature study:
When a new fact does not fit with the reigning theory, one must accept the fact and drop the theory. Claude Bernard
I would argue that when a new fact does not fit, critically examine the fact itself, as well as the theory.
One thing that has always bothered me about the extraordinary claims quote is that it is so highly dependent on subjective judgment. If you have spent your life practicing homeopathy, any claim about its power would not surprise you, so you wouldn't demand extraordinary proof. If you've never used homeopathy, you would view any positive findings more skeptically and would demand more evidence.
It's a bit disillusioning to review this sort of thing, because one side or the other in this debate is severely deluded. On the other hand, it's probably a good thing that people don't drop their cherished beliefs on the basis of one or two studies. If homeopathy really works, evidence for it will gradually rise to the level where it will convince all but the most entrenched minds. If homeopathy really doesn't work, evidence against it will gradually erode the foundation until the only proponents will be again the most entrenched minds.
Further reading