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]

WordNet (r) 3.0 (2006):

theorize
    v 1: to believe especially on uncertain or tentative grounds;
         "Scientists supposed that large dinosaurs lived in swamps"
         [syn: speculate, theorize, 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"

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "theorize":
   analyze, apply reason, conjecture, deduce, entertain a theory,
   espouse a theory, generalize, guess, have a theory, hypothesize,
   infer, intellectualize, logicalize, logicize, philosophize,
   provide a rationale, rationalize, reason, speculate, submit,
   syllogize, synthesize, use reason