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1. To believe especially on uncertain or tentative grounds
   "Scientists theorised that large dinosaurs lived in swamps"
   - speculate, theorize, conjecture, hypothesize, hypothesise [Brit], hypothecate, suppose
    
2. Form or construct theories
   "he thinks and theorises all day"
   - theorize
    
3. Construct a theory about
   "Galileo theorised the motion of the stars"
   - theorize

Derived forms: theorised, theorises, theorising

See also: theorisation [Brit], theoriser [Brit], theory

Type of: anticipate, expect, reason