Many scientists avow that all our thinking is an illusion. Ideas are not measurable but brain activity is.
The funny thing is that all we know of brain activity is its own neuronal or blood flow activity as projected to us by a machine designed to mimic our senses (only more so); those projections are qualia.
Indeed, all we know of the matter of the world is by the qualia it projects, which we are able to perceive. No one has really ever come into contact with matter.
But we know matter exists just the way we know wind exists – by its effects on other matter (which happen to be qualiadelic).