Some part of experience has the quality of being freely determined while the rest is constrained in various degrees and some according to known laws and others by unknown laws. These are differences within experience, but the stuff out of which they are moulded is in all cases the same. To call this 'mind stuff' is going too far, mind being one of the concepts that are usefully employed to distinguish between the types. At any rate there is no need to postulate unknowns outside of all experience.