act on instinct

To do something because one is motivated by a powerful internal impulse. Before I knew what I was doing, I hit the attacker in the face and ran. I just acted on instinct, I guess. If you don't trust someone, get out of that situation immediately. Act on instinct to keep yourself safe. The detective acted on instinct and went to investigate the warehouse by the docks. Sure enough, that's where he found the stolen goods.