The Collaborative International Dictionary of English v.0.48:

Cogitable \Cog"i*ta*ble\, a. [L. cogitabilis, fr. cogitare to
   think.]
   Capable of being brought before the mind as a thought or
   idea; conceivable; thinkable.
   [1913 Webster]

         Creation is cogitable by us only as a putting forth of
         divine power.                            --Sir W.
                                                  Hamilton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cogitable
    adj 1: capable of being thought about; "space flight to other
           galaxies becomes more cogitable" [syn: cogitable,
           ponderable]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "cogitable":
   cerebrate, cogitate, collogue, collude, conceive, connive,
   conspire, contrive, deliberate, devise, envisage, envision,
   imagine, intrigue, machinate, reason, reflect, speculate