The Collaborative International Dictionary of English v.0.48:

Theorist \The"o*rist\, n. [Cf. F. th['e]oriste.]
   One who forms theories; one given to theory and speculation;
   a speculatist. --Cowper.
   [1913 Webster]

         The greatest theoretists have given the preference to
         such a government as that which obtains in this
         kingdom.                                 --Addison.
   [1913 Webster]
   [1913 Webster]

WordNet (r) 3.0 (2006):

theorist
    n 1: someone who theorizes (especially in science or art) [syn:
         theorist, theoretician, theorizer, theoriser,
         idealogue]