The idea has been around for over a hundred years, but nobody has seen one.
You will not see the unconscious mind on a CT, MR, PET or SPECT scan.
It will not pop into view when a neurosurgeon opens the skull.
Like gods and Ptolemeic epicyles it seems to explain anything and everything you want it to.
Like most such myths there is no way to prove it does not exist, but unlike the myth that the world is round, it has not advanced knowledge, even our ability to treat mental illness.
What would believers accept as failure to demonstrate its existence?