Pain
What is pain?
Pain is a mental state, or mental sense. Biologically speaking, pain is a neural sense, because your brain decides to give different scores to the stimuli in conjunction with other variables like your emotions, your frame of mind, your biology, etc. When you have an injury, for example, like banging your hand, your brain gives one score, and my brain gives another score, although the hit is the same.
I know that this is difficult to understand, but what would happen if your brain decides to say that a touch of a feather is like fire in your hand? This occurs with Fibromyalgia. Are those that suffer from Fibromyalgia insane or mad? No, their brains just process the pain in another way, and researchers are trying to understand this process in order to give a treatment.