Through working with Lennie in the course of a day and in seeing how appreciative Lennie was when he received one of Slim's puppies, Slim recognizes that Lennie is not deliberately mean. He understands that the relationship between George and Lennie is one where the former takes care of the latter. Inside the bunkhouse, this becomes evident to Slim in an exchange between George and Lennie:
Lennie sat up quickly. “Give ‘um to me, George.” George said, “You get right up an’ take this pup back to the nest...You take him back or I’ll tell Slim not to let you have him.” Lennie held out his hands pleadingly. “Give ‘um to me, George. I’ll take ‘um back. I didn’t mean no harm, George. Honest I didn’t. I jus’ wanted to pet ‘um a little.” George handed the pup to him.... Lennie fairly scuttled out of the room. Slim had not moved. His calm eyes followed Lennie out the door. “Jesus,”he said. “He’s jus’ like a kid, ain’t he?”
This excerpt is important in detailing Slim's understanding of Lennie for a couple of reasons. The first is that Lennie immediately reverts back to a child when George threatens to take the puppy away from him. When Lennie pleads, "Give 'um to me," and says "I didn't mean no harm," it becomes clear to Slim that Lennie has an approach to life like a child and is not deliberately mean or cruel. Lennie does not challenge George's authority, but rather capitulates to it as a child would. Slim notices this. At the same time, Lennie's submissiveness when confronted proves to Slim that Lennie is "like a kid." Slim has seen men like Curley who use violence and power to achieve what they want. Slim has seen Lennie work. He knows the physical capacity he possesses. Yet, Slim recognizes that Lennie does not use any of that with George and even when he does not get what he wants, he does not revert to his physicality. Rather, Lennie acquiesces to George. It is through Slim's interaction with other men that he realizes Lennie's differences and understands that he is more of a child than anything else.