WordNet (r) 3.0 (2006):

hypothesize
    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]

Moby Thesaurus II by Grady Ward, 1.0:

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