n.

1. a rational motive for a belief or action;
   ground
   
   • the reason that war was declared
   • the grounds for their declaration
2. an explanation of the cause of some phenomenon
   
   • the reason a steady state was never reached was that the back pressure built up too slowly
3. the capacity for rational thought or inference or discrimination;
   understanding, intellect
   
   • we are told that man is endowed with reason and capable of distinguishing good from evil
4. the state of having good sense and sound judgment;
   rationality, reasonableness
   
   • his rationality may have been impaired
   • he had to rely less on reason than on rousing their emotions
5. a justification for something existing or happening;
   cause, grounds
   
   • he had no cause to complain
   • they had good reason to rejoice
6. a fact that logically justifies some premise or conclusion
   
   • there is reason to believe he is lying

v.

1. decide by reasoning; draw or come to a conclusion;
   reason out, conclude
   
   • We reasoned that it was cheaper to rent than to buy a house
2. present reasons and arguments;
   argue
3. think logically
   
   • The children must learn to reason