> This consciousness that you're sure you possess, but you can't measure, detect, define, or even really describe?

But you can measure consciousness in humans, for example anesthesiologists do it all the time! Do you see difference between being asleep (without dreaming) or being under anesthesia and not being asleep or under anesthesia? If yes then that difference is consciousness, it’s something you yourself can experience. I think you just mix multiple things, perhaps you think consciousness is free will or something like that otherwise I can’t explain how can you hold this position. If you are thinking right now then you are conscious hence consciousness exists. And that’s it.

> It's a nonsense idea, primitive and inherited from religion

No, consciousness is something you just experience. Don’t you have inner life, thoughts, experiences? You ascribe some magic properties to consciousness and then - since magic doesn’t exist - come to conclusion that it can’t exist. But you know, you should just skip the magic part.