> To continue your thought experiment, another researcher designs a new, double-blind trial of gourds, and discovers that while gourds per se have no effect, the placebo effect is statistically significant.
Without a search for causes, for explanations, even the placebo effect is routinely disregarded. For example, it has been recently discovered that all psychological therapies are equally efficacious. Until now, the assumption was that Cognitive-Behavioral Therapy was superior to others, but that's been disproven. But, even though all therapies produce the same outcome, no one in psychology seems willing to consider the idea that it's all placebo effect.
> In fact, any explanation at this stage would be provisional and highly suspect.
Except the one that Occam's razor suggests, the default assumption under these circumstances: placebo effect. Or, perhaps better, the non-explanation suggested by the null hypothesis -- nothing meaningful has been measured and no conclusions can be drawn, which I think is your point.
> What are we to do, ignore this phenomenon as if it didn't exist?
No, but as scientists, we would do well to avoid drawing any conclusions not supported by rigorous experiment -- including the responsibility to propose and then test a theory about what's been observed.
