But we can. We can explain what wavelength of light we see as red, and explain to them (in terms from very basic physics) what other colors we see as red. They could then correlate that claim to apples, strawberries, red roses etc. And the aliens could then show US lots of red objects, and we would both agree on the color. Likewise, if you could say "thoughts emit weak EM at 3.75GHz" and could then track that emission both during life and after death, you'd have a strong argument that thoughts persist. But every indication we have of thought (action potentials, dynamic neurotransmitter gradients, firing rates etc) ceases after death - so the straightforward conclusion is thoughts cease as well. Let's take another example. Let's say you have an LED light bulb. You turn it on. It goes on. You turn it off. It turns off. You conclude it is a device for turning electrical power into light. "No," I say. "The light persists after you turn it off, so it's not converting electricity to light; it's converting manna to light." "What's manna?" you ask. "No one knows," I say. "But the light is off!" you state, pointing to the now-dark bulb. "It's really still on; you just can't see it," I say. Is that a reasonable argument?