Things are made out of particles of plastic and quarks of cardboard. Thoughts are made out of edited memories and amateur prophecies.
Things are stable. If you put down a teapot and turn away, behind your back it will still be a teapot.
But if you turn away from a thought it will never be the same when you come back to it. It will be darker or brighter, or less important. Or scarred because you've thought it too many times.
Is there any place in the universe more unstable than the inside of our heads?
Are there no drugs powerful enough to tame the turbulent flow of our perpetual mental alchemy?
You can think about lead. And then you can think about gold.