The Collaborative International Dictionary of English v.0.48:

Cognoscible \Cog*nos"ci*ble\, a.
   1. Capable of being known. "Matters intelligible and
      cognoscible." --Sir M. Hale.
      [1913 Webster]

   2. Liable to judicial investigation. --Jer. Taylor.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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