The author says,
"Slim's opinions were law."
He makes this comparison between Slim's opinions and law when the men are debating whether Candy should have his dog euthanized. Candy, who is very attached to his dog and has had him for a long time, does not want to let him go, but the other men are bothered by his smell, and argue that the dog, who is very old and arthritic, would be better off dead and gone. When Slim weighs in with his opinion, that the dog should be put out of its misery, Candy knows that the decision has been made for him. He looks helplessly at Slim, knowing he is beaten, because the men consider Slim's opinion to be law.
Slim is a smart and capable man, "a jerkline skinner, the prince of the ranch." He is a man of quiet wisdom and authority, whose eyes are described as "God-like;" there is "a gravity in his manner and a quiet so profound that all talk stopped when he spoke." Slim's "word (is) taken on any subject, be it politics or love...his ear hear(s) more than (is) said to him, and his slow speech (has) overtones not of thought, but of understanding beyond thought." It is to Slim that George reveals the story of how he and Lennie have come to be traveling together, and it is Slim who fixes it so that Curley, whose hand has been crushed by Lennie in a confrontation instigated by Curley himself, will not tell what has happened and get Lennie in trouble. It is also Slim who understands the gravity of what has happened when George shoots Lennie at the end of the story, and tries to comfort the distraught man.