Rational v. Irrational

Rational:

-adjective
1. agreeable to reason; reasonable; sensible: a rational plan for economic development.
2. having or exercising reason, sound judgment, or good sense: a calm and rational negotiator.
3. being in or characterized by full possession of one’s reason; sane; lucid: The patient appeared perfectly rational.
4. endowed with the faculty of reason: rational beings.
5. of, pertaining to, or constituting reasoning powers: the rational faculty.
6. proceeding or derived from reason or based on reasoning: a rational explanation.

Synonyms: all there*, analytical, balanced, calm, cerebral, circumspect, cognitive, collected, cool*, deductive, deliberate, discerning, discriminating, enlightened, far-sighted, impartial, intellectual, intelligent, judicious, knowing, level-headed, logical, lucid, normal, objective, perspicacious, philosophic, prudent, ratiocinative, reasonable, reasoning, reflective, sagacious, sane, sensible, sober, sound, stable, synthetic, thinking, thoughtful, together, well-advised, wise
*denotes Slang

Irrational:
-adjective
1. without the faculty of reason; deprived of reason.
2. without or deprived of normal mental clarity or sound judgment.
3. not in accordance with reason; utterly illogical: irrational arguments.
4. not endowed with the faculty of reason: irrational animals.

Synonyms: aberrant, absurd, brainless, cockamamie*, crazy, delirious, demented, disconnected, disjointed, distraught, fallacious, flaky*, foolish, freaky, incoherent, injudicious, insane, invalid, kooky*, loco, loony*, mad, mindless, nonsensical, nut job, preposterous, raving, reasonless, ridiculous, senseless, silly, sophistic, specious, stupid, unreasonable, unreasoning, unsound, unstable, unthinking, unwise, wacky*, wild, wrong