reason-n; 2 Senses

Sense Number 1: grounds

Commentary: cause, justification - a rational motive for a belief or action; a justification or explanation for something existing or happening

The reason on that war was declared.
The grounds for their declaration.
He had no cause to complain.
They had good reason to rejoice.
The reason a steady state was never reached was that the back pressure built up too slowly.
There is reason to believe he is lying.

WordNet 2.0 Sense Numbers: 1 2 5 6

Sense Number 2: reasonableness

Commentary: understanding, intellect, rationality - the capacity for rational thought or inference or discrimination

We are told that man is endowed with reason and capable of distinguishing good from evil.
His rationality may have been impaired.
He had to rely less on reason than on rousing their emotions.

WordNet 2.0 Sense Numbers: 3 4