Ch. 11 Part 1: No Crime Committed. Key TermsAllibi. Ch. 11 Part 1: Why Do I Need To Know This?. Because in the event you are investigated for a crime you did not commit, you will need to know how to prove your innocence.Because there may come a time when you will have to do things that are questionable and you will want to know if you have a legal defense for doing so.Because if you want to make a valid argument, you need to know how to present a defense to a case..