- The brain is a part of the central nervous system
- The brain can control animal body through the nervous system
- So, the brain is a part of the nervous system that controls the body of an animal

- A body system is a part of the body / an animal
- The nervous system is a kind of body system
- Therefore, the nervous system is a part of the body of an animal

- The brain is a part of the nervous system that controls the body of an animal
- The nervous system is a part of the body of an animal
- Hence, the brain is a part of the nervous system that can control the nervous system

- An organ is a part of a body system
- If a part of a system can control that system, then that part is a major part of that system
- Hence, if an organ in a body system can control that body system, then that organ is a major organ in that body system

- The brain is a part of the nervous system that can control the nervous system
- If an organ in a body system can control that body system, then that organ is a major organ in that body system
- Therefore, the brain is a major organ in the nervous system

Hence, the brain is a major organ in the nervous system
Question: Which is a major organ of the nervous system?
Context: - An organ is a part of a body system 
- A body system is a part of the body / an animal 
- The nervous system is a kind of body system 
- If a part of a system can control that system, then that part is a major part of that system 
- The brain is a part of the central nervous system 
- The brain can control animal body through the nervous system