If I had a tape measure that I knew had accurately recorded lengths and I measured my table to be three foot long I'd trust it without further evidence.
Some tape measures are not accurate. If we knew, that certain tape measures were sometimes out by a factor of ten and I told you my table was three feet long you'd think it was between 3.6 inches to 30 foot, pending further information.
Now, if I had a device that detected rain placed in my garden but that device recorded false positives 1% of the time, I could say that it was a scientific piece of evidence for it having rained if it recorded that it rained whilst I was away.
In these cases we know the error margins.
What if my device was manufactured by Acme corporation? Acme are devils and some of their devices register false positives as much as 100% of the time, sometimes as few as .0001% of the time. One cannot know which one it is until you had tested it.
If I had tested my machine and recorded that it has over a period of 5 years never recorded a false positive on its rain count for any day, I can be confident that my positive result for rain means that it did in fact rain whilst I was away.
That would be scientific.
One final step. If I didn't bother to test my device by observing it for 5 years and I bought another Acme device that connects to the first and tells me how accurate it was I would find myself in a similar position to the rest of the world with the wife scenario.
The second device can produce false results occasionally as well you see. The second device says the first is reliable and this is a valid piece of scientific evidence. It is less likely that both the devices are unreliable, but not impossible.
In analogy terms my wife's brain is the first device and my brain is the second device.
As with any scientific evidence only relying on two measurements is hardly good science (especially when both evidences are using
Acme devices human brains). There is still a good chance we're getting false positives. Our conclusion is highly tentative. The more evidences we have (and the more evidence we have that the evidence is reliable evidence) the more certain we can be that the conclusions we draw from the evidence are accurate.