be of sound mind

1. To have full, unimpaired thought, memory, and cognitive understanding; able to think and understand for oneself. My grandfather may be in a wheel-chair at 94, but he's still of sound mind—sharper than a lot of people I know, actually! A: "Have you heard some of the orders he's been giving lately?" B: "I know, he doesn't seem to be of sound mind to me." You can't tell people you want to start a business selling bees as pets—they'll think you aren't of sound mind anymore!

2. law By extension, to be fully mentally competent and thus able to stand trial or be otherwise legally responsible. The state-appointed psychiatrist determined that the defendant was not of sound mind when signing the contract, and thus is not legally bound to honor the terms therein. I doubt that Travis was of sound mind when he committed that crime. You could be held legally liable if the judge rules that you were of sound mind at the time of the fire.