But evidence of absence is, and in this case we have a lot.

For the last 400 years, pathologists on every country had filleted and put, lets say tens thousands of human brains and human guts under the microscope. One of them has systematically a microbiome, easy to see. The other don't, except when is diseased or rotten. The sample token here is huge, maybe millions.

If we would had searched 400 years for this chocolate teapot without finding it, we could conclude with a solid suspicion that there is not such thing.

This is very different than just saying "I don't think that there is bacteria in the brain but I never searched for it". All pathology science is based in searching for it. We created gram staining dyes, scanners, tags, gold coated plates for electronic microscopes, DNA analysis... exactly for that.

If there really is a microbiome living in each healthy brain, we should have found it 150 years ago.

> For the last 400 years, pathologists on every country had filleted and put, lets say tens thousands of human brains and human guts under the microscope. One of them has systematically a microbiome, easy to see. The other don't, except when is diseased or rotten. The sample token here is huge, maybe millions.

Is that really the case? By my understanding of the article, we find plenty of bacteria whenever we look at human brain samples. The problem is that it's very hard to tell if that bacteria was already present in the brain, or if it got in through the process of cutting the brain open (especially by contamination with other tissues), or if it was indeed present before the procedure, but only because the individual was very old or had a disease.

Yeah exactly. It’s not an unreasonable search and we don’t have confidence our search methods work. Hell, the Ryugu sample was contaminated while in a hermetically sealed clean room filled with nitrogen gas. Either the blood brain barrier is even more effective or maybe the story isn’t quite so clear. This is not an unreasonable hypothesis nor do have we exhausted search. Hell, we’re literally talking about it in response to a related find in another species. So it’s definitely not a wild theory or one that conflicts with known theories.

The chocolate teapot example is a non sequiter as it fails both Occam’s razor and the principle that extraordinary claims require extraordinary evidence not to mention that it wouldn’t follow any laws of known science and it’s existence very well would upend quite a few of those. The scientific method isn’t something you get to apply piecemeal.