When you say to yourself, "What is seven times nine?" and then "sixty three" pops up, you are certainly conscious of thinking "sixty three." So that's definitely mental; and so is the brain state that corresponds to your thinking "sixty three." But what about the brain state that actually found and delivered "sixty three"? You are certainly not conscious of that, although you were just as conscious while your brain was finding and delivering "sixty three" as while you were breathing, though you don't feel either of those states.
We can agree that consciousness emerges from a sufficiently complex system, but not from insufficiently complex systems. While the metaphysical doubt that a rock could be somehow conscious, or a tree, or Gaia, always remains. . .it is merely a qualification made to preserve intellectual honesty. The doubt is really reserved for things like biomes and planets, not for dust and bushes. It's subjective, sure, but it's the best we've got.
This question has been addressed so often that the language for discussing it is well established. It is possible there are just things that cannot be something, kind of like how "0" and "zero" are things that represent nothing. It's a paradox, not an inconsistency.
Anywho, the really interesting development is that as we offload cognition into artificial actors we are accumulating context for the discussion that was impossible before the microchip. New innovations are being created every day that do things we previously associated only with conscious actors. Since we do not consider these new mechanisms conscious, we can no longer say those functions are conscious. If a function can be provided by purely vegetative processes then consciousness must be something else.
Consciousness is one of those leading-edge concepts because everything we've nailed down as mere complexity, so far, has failed to explain it. Like how the round Earth was just a theory until someone actually managed to sail all the way around it, because the surface that had been explored up to that point didn't fully explain the Earth's roundness. I think we'll figure it out eventually. . .probably a few seconds after SkyNet becomes conscious and tries to kill us all. . .but life's a journey, not a destination.