Who can you trust?

I am supposed to give a speech in a couple of days about how to recognise who you cannot trust...this will be the highlights.
It is a difference between trusting people in ordinary life and trust people from a security system standpoint. Why? Because on the Internet you will never know the people that tries to access your system. If the systems are accessible for the people you know (and trust) they are open for everyone else as well...and can you really trust everybody you know...and do you really know them?
Think about it for a while, even if you trust anyone that works for you, you have to build a security system that protects you from the people that you do not know. If you do this properly you will end up with a system that also protect you from actions from people that you thought you could trust.
Conclusion: Forget about the discussion about who, you have to treat everybody equal.