[Stupulous]

Is math an illusion? Math is a property of systems, as is our and our calculators ability to do it. I would argue that makes it more real.

When you see an object in reality, you're really making a series of assumptions. What you see is reflected light, and you construct a highly innacurate but useful model of what that light logically implies. Something that is external can only be perceived epistemically, as a sufficiently good guess. There's no evidence we're not in a simulation, so it's a toss-up as to whether anything physical exists at all.

Contrast the concept of self awareness. 'I think, therefore I am' is not a guess. It is a certainty, regardless of the broader context, that somewhere, something is a host to my experiences and thoughts.

Tangent: You can extend that a little bit by reasoning that you exist, therefore the things you interact with must exist as something. That doesn't get you past the possibility of the matrix, but it does escape total nihilism.