When we say someone is "in his right mind," we imply that he is sane, rational, and capable of making sound decisions. However, there are several synonyms that can be useful in certain contexts. For instance, we might say that someone is "of sound mind" or "in clear mind" to stress their mental stability. Alternatively, we might use phrases such as "mentally lucid" or "cognitively sound" to describe someone's mental state as sharp and focused. Other synonyms could include "well-balanced," "mentally stable," or "not deranged." Whatever the words used, all these terms capture the sense that someone is thinking clearly and is capable of making rational judgments.