Jesus v. the words of demons

I've wondered how literally to take references to demons in the New Testament: are they living spiritual beings, or only how the ancient world understood or talked about certain mental illnesses? I don't have any special insight into that question, and this post could be relevant to people who hold either view. 

Regardless of what you think of demons, they have a reputation for getting in our head with crafty words. The Bible portrays demons or other similar beings as spreading doubt, temptation, and lies. While they may work to increase doubt, fear, lust, or pride, still their tool of choice is words. In today's reading from the accounts of Jesus' life, 

He cured many who were sick with various diseases, and cast out many demons; and he would not permit the demons to speak, because they knew him. (Mark 1:34)

Jesus' brother James said that the demons believe in God -- and tremble. There is a type of "fear of God" that is incompatible with faith, as James pointed out, a demonic fear. Demons have a reputation for lying, and even if they speak a truth it would not be used truthfully. 

In my own life, I have not known literal demons (as far as I know). But in our day we still say things like "he's wrestling with his demons" about someone who is fighting a spiritual battle inside. So I have known what people of this day call demons, as far as troubling and destructive thoughts that persist and take on "a life of their own." It may be helpful for me to remember that demons are known to lie, that even if they tell a truth they do not tell the truth. When troubling thoughts persist, it might be helpful for me to remember that when Jesus healed people, he did not permit the demons to speak.