Which preposition to use with undemonstrable

by Occurrences 1%

There is the intrusion of the cheap atheist, the small materialistic thinker, into a sphere of which certainly no clergyman or priest has any monopoly, that sphere of what we call the spiritual life, which, however undemonstrable by physical tests, has been real to so many men and women whose intellects can hardly be called negligible, from Plato to Newman.

Which preposition to use with  undemonstrable