The substrate of mind itself is not stuff: it is the subject, not an object. It is the medium from which perceptions arise, but is itself not perceivable for exactly the same reason that the eye that sees cannot see itself without a mirror; or – as Alan Watts put it – that you can’t bite your own teeth.96 As such, the substrate of mind cannot be measured, detected, or analyzed like some kind of stuff, because it is that which measures, detects, and analyzes in the first place. The substrate of mind is not a material, but that which imagines all materials.