What a belief forbids reveals whether it's real
Asking "what does this belief predict?" is good. Asking "what does it prohibit?" is better.
Élan vital supposedly explains what makes living things alive. What would definitely prove it wrong — what experience would falsify it? The question produces a blank. A belief compatible with any possible observation carries no information about the world. It floats.
A belief that genuinely models the world has to make some experiences less likely or impossible. If it doesn't rule anything out, it doesn't say anything. Vividness and confidence don't help here — a belief can feel substantial while permitting everything.
The prohibition test is sharper than the prediction test because it's harder to pass. Almost any belief can be made to "predict" something vaguely. Fewer can name what they'd definitively rule out.
If you can't name an experience your belief would rule out, it's probably floating. Evict it.
Source claim: The strongest test of a belief's reality is identifying what experience it would rule out — a belief compatible with any outcome says nothing about the world.