The Collaborative International Dictionary of English v.0.48:

Incognizable \In*cog"ni*za*ble\, a.
   Not cognizable; incapable of being recognized, known, or
   distinguished. --H. Spenser.
   [1913 Webster]

         The Lettish race, not a primitive stock of the Slavi,
         but a distinct branch, now become incognizable.
                                                  --Tooke.
   [1913 Webster]

WordNet (r) 3.0 (2006):

incognizable
    adj 1: incapable of being perceived or known [syn:
           incognizable, incognoscible]