It depends on how you define "illusion." Psychologist and neuroscientist Michael Graziano developed his Attention Schema Theory of consciousness to explore this question. Essentially, he argues that the human brain evolved an oversimplified model of how it processes sensory input and directs attention (called a schema) so as not to get overwhelmed by the physical details of what’s happening. Graziano suggests that our subjective inner experience (or consciousness) only feels nonphysical, because over the years, our brains have adopted schemas as a type of mental shortcut, so we’re not aware of all the physical processes that are actually occurring as we have firsthand experiences in the world.