Lennie, the keeper of the dream in Of Mice and Men by John Steinbeck, is, indeed, simple-minded and childlike. At the river outside Soledad, George must repeat again and again to Lennie where they are going. When George complains that he must always repeat, Lennie says softly,
"Tried and tried [to remember]...but it didn't do not good."
After George takes the mouse from Lennie that is dead, he promises to let Lennie have a fresh one sometime. Lennie says as he hangs his head dejectedly,
"I don't know where there is no other mouse. I remember a lady used to give' em to me--ever'one she got. But that lady ain't here."
George scoffed. "Lady, huh? Don't even remember who that lady was. That was your own Aunt Clara.
In another instance, Lennie's inability to reason is very apparent when Crooks taunts him after Lennie and Candy enter his quarters in the stable. in his jealousy of their friendship, Crooks asks Lennie what he would do if George went into town and never returned.
Crooks' face lighted with plesure in his torture. "Nobody can't tell what a guy'll do," he observed calmly. "le's say he wants to comeback and can't. Spose he gets killed or hurt so he can't come back."
Lennie struggled to understand. "George won't do nothing lke that," he repeated. "Geore is careful. He won't get hurt. He ain't never been hurt, 'cause he's careful."
But Crooks continues his bantering, until in his miscomprehension, Lennie becomes angry:
"Want me ta tell ya what'll happen? They'll take ya to the booby hatch. They'll tie ya up with a collar, like a dog."
Suddenly Lennie's eyes centered and grew quiet, and mad. He stood up and walked dangerously toward Crooks. "Who hurt George?" he demanded.
I love Lennie, but frankly just about anything he says demonstrates his simple-mindedness. This is a novella (a very short novel), so John Steinbeck doesn't have much time or space to create his characters in a way the reader will find believable and relatable. With Lennie, he creates an impression by how he looks, what he does, and what he says from the very first time we meet Lennie.
Lennie has flopped down at the edge of a pool of water and doused his entire head, hat and all. He's thirsty and the water is refreshing, but this image of a grown (extra large) man pulling his head out of the water, dripping all over himself, is comical. Then he speaks:
"'Tha's good," he says. 'You drink some, George. You take a good big drink.' He smiled happily."
The image then changes from comical to that of a child who wants to share something he really likes.
Another line which Lennie speaks is this:
"'I don' like this place, George. This ain't no good place. I wanna get outa here.'"
He is referring to the ranch (and the barn with Curly's wife specifically), and it turns out he's right; however, his is not a logical, well reasoned argument. Instead, it's more like a child who just senses something bad but can't really articulate what's bothering him.
A third quote is:
"'I done another bad thing.' It don't make no difference.'"
Lennie has killed a woman--by accident, of course--and all he knows to say is he's done "a bad thing" once again. That's not the reaction of a grown person; it's the confession of a child.
Lennie's a child trapped in a man's body, and nearly everything he says and does confirms that.
Lennie's simple mindednesses in Of Mice and Men can be see through the following three quotes:
Lennie went back and looked at the dead girl. The puppy lay close to her. Lennie picked it up. "I’ll throw him away," he said. "It’s bad enough like it is."
Lennie's simple mindedness is displayed in his inability to differentiate between right and wrong. His frame of reference is based solely upon George's reaction to a situation. Lennie realizes that the dead girl and the puppy laying their will make George mad, therefore, in Lennie's simple mindedness, he believes that removing the puppy will remedy the situation and never takes into account the dead girl. He demonstrates his inability to comprehend that a dead girl and a puppy are not of equal value within the context of society.
"You ain't gonna leave me, are ya George?"
Lennie's simple mindedness is displayed through his child-like innocence. He does not comprehend the consequences of his actions and looks to George to be his faithful protector.
"I remember about the rabbits, George."
George has a dream of owning a farm, but he knows that the road to fulfilling the dream is going to a hard one, full of obstacles and hardships. Lennie's simple mindedness allows him to believe that the dream of owning a farm will happen. Lennie has complete faith and trust in George's words. His simple mindedness allows him to have the child-like faith and be oblivious to the long road ahead.