Men's rea looks at what the criminal was thinking at the time of committing the crime. Most crimes require criminal intent. This is to punish those who are most dangerous with the rationale being that they will be more likely to do bad things again rather than the person who does so accidentally. However, sometimes you can be responsible even when men's rea doesn’t exist in order to protect other people. Note: there are exceptions and not all crimes require men's rea.