unprovable (adj.)

• agnostic
• ambiguous
• capricious
• chancy
• changeable
• controvertible
• dicey
• doubting
• equivocal
• erratic
• fickle
• hesitant
• hesitating
• incalculable
• indecisive
• irresolute
• polysemous
• skeptical
• touch-and-go
• unaccountable
• uncertain
• unconvinced
• unforeseeable
• unpersuaded
• unpredictable
• unsupportable
• unsure
• unverifiable
• variable
• wavering
• whimsical