The human brain or rational
The rational brain is the part of the brain that developed later at evolving and is involving and somehow controls the other two brains, the oldest and most instinctive.
The functions of the brain are the creativity, imagination, problem solving, reasoning and reflection, kindness, empathy, compassion, responsibility, etc..
It is this brain that differentiates us from other animals, which is what gives us the ability to reason (which is not simply think, because animals also think, but the ability to think about what we have thought or what we have done before.
It is therefore the brain that allows us to evolve as a species.
The three brain must interact
Once explained one might think the human brain is the command to be provided and must cancel the other two, which are more aggressive, instinctive and dangerous.