WordNet sense 1 (the state of having good sense and sound judgment):

rationality, reasonableness

WordNet sense 2 (a rational motive for a belief or action):

ground

WordNet sense 3 (a justification for something existing or happening):

cause, grounds

WordNet sense 4 (the capacity for rational thought or inference or discrimination):

intellect, understanding

WordNet sense 5 (present reasons and arguments):

argue

WordNet sense 6 (decide by reasoning; draw or come to a conclusion):

conclude, reason out