The Collaborative International Dictionary of English v.0.48:

Knowable \Know"a*ble\, a.
   That may be known; capable of being discovered, understood,
   or ascertained.
   [1913 Webster]

         Thus mind and matter, as known or knowable, are only
         two different series of phenomena or qualities. --Sir
                                                  W. Hamilton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

knowable
    adj 1: capable of being known [syn: knowable, cognizable,
           cognisable, cognoscible] [ant: unknowable]