Why is your brain a defensive tool?
Your brain is your best weapon. It has been optimized by evolution to keep you alive. It is always with you, ready to be used, and you can exercise it without leaving your chair.
Your brain creates your attitude, which is one of the most important survival characteristics you can possess. If you believe, deeply and truly, that you are worth protecting, and if you project that attitude to those around you, most criminals will simply leave you alone.
The best self-protection for most crimes is prevention, and you can easily prevent many crimes just by being aware (using your eyes, your ears, and yes, your brain) of who and what is around you. If prevention fails, and you have to use some of the other tools and techniques here, your brain will automatically bring forth the physical moves you have practiced.
A strong, survival-oriented attitude, coupled with some specific physical training is a devastating combination that will triumph over someone who tries to harm you.