What do people mean when they say there's no evidence?

It's interesting that people on the web constantly say "there's no evidence for an afterlife", or "there's no evidence for psi". For this seems to be straightforwardly false. Take reincarnation. There's research into alleged past lives of young children that appears to show that they are getting accurate information about a person that had previously lived. Or take psi. Parapsychologists have provided plenty of evidence for it. 

Obviously, most of these people are presumably just repeating each other. But what justification could be given for the origin of this claim that there's no evidence for such things?

Consider the case where someone declares that we have evidence that beds are dangerous as people disproportionately die in them compared to elsewhere. Would this be evidence that beds are dangerous, even if only very weak evidence?

It depends on what people mean by "evidence". I would say yes, but I think most science educated people would say no. One reason why they would say no is because we already have a good explanation for why people die in beds -- namely because they go to bed when they are ill. But, another more compelling reason, is that it seems there's no conceivable mechanism whereby beds in and of themselves could somehow play a causal role in precipitating death.

Let's, for the sake of argument, accept for the moment these peoples' conception of "evidence" as requiring a possible mechanism.  Now, consider reincarnation. Even though young children appear to recollect previous lives, people assert there's no conceivable causal mechanism whereby consciousness can depart from a person who has died, to then appear in a fetus or new born baby. Hence, until someone proposes a possible mechanism, there can be no scientific evidence for reincarnation. Same applies more generally to an afterlife, or to psi.

There's a deep problem here though. There appears to be an implicit assumption that our everyday embodied consciousness being produced by brains is entirely unproblematic and hence that there will be a mechanism whereby this is achieved even though we have yet to discover it. Or, even if it's not entirely unproblematic, we can be confident that brains do somehow produce consciousness because of all the ways that brains affect our mind-states -- especially when we consider such debilitating conditions as dementia.

I would maintain, though, that the mind-brain correlations that supposedly prove that brains produce minds, appears to be no more persuasive than eyeglasses- acuity of vision correlations prove that eyeglasses produce vision. In the case of the eyeglasses, there clearly is no conceivable mechanism. And we know that vision already exists, it is merely changed by the eyeglasses. However, it seems to me, brains are in the same boat. There is nothing about the ultimate particles that brains are made of that could possibly produce our phenomenological experiences. So, similar to eyeglasses-vision, perhaps consciousness already exists with brains merely changing it.

But, regardless of whether I am right or wrong about the brain somehow producing consciousness, it seems to me we don’t have any conceivable mechanism for how the brain does this. Hence, if despite the correlations we have no evidence that beds precipitate death, then likewise, despite the mind-brain correlations, and since we lack a conceivable mechanism, we also have no evidence that brains produce consciousness. So even if – contrary to my position – the thesis that brains produce consciousness is at least conceivable (unlike eyeglasses producing vision), we have no evidence that brains actually do so (least of all do we have proof).

Indeed, we have a more plausible scenario; namely consciousness exists all along with brains merely changing it i.e. consciousness is fundamental. And if that should be the case, then consciousness can exist independently of brains just as vision exists independently of eyeglasses.  If this is so then the demand for a mechanism is misplaced.  Indeed, demanding a possible mechanism appears to presuppose that consciousness is material and hence is question-begging.

Now let’s consider psi. Even if the notion of the brain producing consciousness is unproblematic, the fact still remains we have no causal mechanism for how it is produced. But psi, if it exists, will be an ability or property of consciousness. If we lack a causal mechanism for the very existence of consciousness, how on earth can we expect anyone to produce a causal mechanism for psi? Further, if I am correct and consciousness is not produced by the brain, how can we a priori declare what properties consciousness must have?

When people say psi is impossible due to no possible causal mechanism, they have in mind that physics pretty much describes the entirety of reality and it doesn’t allow for psi. But physics leaves out consciousness in its description of reality; indeed it suggests consciousness shouldn’t exist. If it suggests this, it is scarcely likely to suggest a property of consciousness like psi should exist, or even free will or a causal role for consciousness.