Source : Webster's Revised Unabridged Dictionary (1913)

Ratiocinate \Ra`ti*oc"i*nate\, v. i. [L. ratiocinatus, p. p. of
   ratiocinari, fr. ratio reason. See {Ratio}.]
   To reason, esp. deductively; to offer reason or argument.

Source : WordNet®

ratiocinate
     v : reason methodologically and logically