The adjective rational means having or exercising the ability to reason.
The noun rationale refers to an explanation or basic reason.
- "No rational argument will have a rational effect on a man who does not want to adopt a rational attitude." (Karl Popper)
- The senator challenged the government's rationale for the financial bailout.
(a) What is the mayor's ____ for trying to sell three of the city's public hospitals?
(b) "It is not to be forgotten that what we call _____ grounds for our beliefs are often extremely irrational attempts to justify our instincts." (Thomas Huxley)