Following up on a tip that there is drug trafficking going on in a particular apartment, you knock on the door and ask if it is okay to enter. The person who answers the door gives you permission and even says it's okay to look around. Under the couch cushions in the living room you discover a baggie containing illegal drugs. You arrest all residents of the apartment for possession. At the trial, the defense attorney argues that the person who answered the door was only a roommate and did not have the authority to give permission for the search without all other residents being present. This argument is dismissed on what principle?