In these sections, Lennie is presented as dumb. I do not say this to criticize him, but in both sections his behaviors are linked to animals. For example, in section 1, Lennie is compared to a bear. As he reaches his hand down to the water, Steinbeck describes his "bear paw". In section 6, Lennie imagines the great big rabbit that is talking to him. Both intances of the use of animals suggest two principles to me. First, I believe this demonstrates Lennie has arrived at the most basic, simple place in life: he deals with needs particularly for survival. Second, this illustrates the affinity he had for comfort. He longed for soft things. When he couldn't physically reach them, his mind and manner naturally gravitated toward them. Thus, I would say you could call Lennie primitive because that word is used to describe both the features of dumb and animal-like.
Lennie is also presented as child-like. Children like animals (just as we discussed his desire for them). Children also long for boundaries. In section 1, George established boundaries and rules. In section 6, both Lennie and George followed-through with the consequences of the boundaries set. Lennie needs George as a father-figure to make decisions for him. This occured in the first section as the two of them crafted their plan to "live off the fatta' the lan'" and in the last section as George made a most difficult decision to spare Lennie from pain.