The Collaborative International Dictionary of English v.0.48:

Theorize \The"o*rize\, v. i. [imp. & p. p. Theorized; p. pr. &
   vb. n. Theorizing.] [Cf. F. th['e]oriser.]
   To form a theory or theories; to form opinions solely by
   theory; to speculate.
   [1913 Webster]