Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

theorize
     v 1: to believe especially on uncertain or tentative grounds;
          "Scientists supposed that large dinosaurs lived in
          swamps" [syn: {speculate}, {theorise}, {conjecture}, {hypothesize},
           {hypothesise}, {hypothecate}, {suppose}]
     2: construct a theory about; "Galileo theorized the motion of
        the stars"
     3: form or construct theories; "he thinks and theorizes all
        day"