The phrase "out of reason" implies something that is illogical or unreasonable. To describe the opposite of this, one could use antonyms such as "logical," "practical," "rational," or "sensible." These words suggest that a decision or action is based on sound judgment and common sense. Other antonyms for "out of reason" might include "justifiable," "defensible," or "legitimate," which indicate that a particular course of action is reasonable and justified. By using antonyms for "out of reason," we can effectively communicate the opposite of something that is irrational or nonsensical.