Comment by p_noumenon

Consciousness is literally the only thing which can't be an illusion, because it's through consciousness that illusions are perceived in the first place. Cogito ergo sum; you can doubt what you are experiencing as much as you like, maybe it's a dream, maybe it's a simulation, maybe there's an evil demon deceiving you through magical means, but you can't doubt the fact that you are experiencing at all, even doubt itself is part of the experience.