The phrase "of sound mind" is often used to describe someone who is mentally capable or rational. Synonyms for "of sound mind" include "mentally sound," "in good mental health," or "of sane mind." Other synonyms may include "lucid," "coherent," or "levelheaded." These terms may be used to describe someone who is capable of making rational decisions and thinking clearly. Additionally, phrases such as "with a sound mind" or "possessing a sound mind" may also be used as synonyms for "of sound mind." Ultimately, these phrases all convey the same idea of mental clarity and stability.