State of mind = your attitude and way you are thinking at the time. Good state of mind = happy, positive, cheerful Bad state of mind = sad, pessimistic, mad Being in fear is a state of mind. A state of mind is having a certain emotion. It is a common English expression. If I said, " I have a solid state of mind" it would mean I feel good and can think clearly

In the example "fear is a state of mind" it suggests that fear is something you only experience in your head (yoru mind) and can therefore be overcome. It would be something you said to someone when they are feeling scared or nervous and you want to give them the power to do whatever it is they are fearing. There is nothing physically stopping them. A common way to use "state of mind" would be when someone has gone through a traumatic incident. When talking about them you could ask, "what state of mind is he in?" - meaning "how is he feeling?"