I believe that there are demons in our world that try to influence our thoughts and decisions. To be clear, I don't believe that this makes us not responsible for our actions. Also, I do not confuse the influence of these demons with legitimate mental illnesses. That said, we are in the middle of a cosmic battle with our very souls at the center.
How do we fight something we can't identify with our senses? The answer is simple and yet terribly difficult at once: live the best life that you can. Jesus has already given us power to cast these demons out. When tempted to do something contrary to what I know is right, or when I find my thoughts drifting into the realm of temptation, I can identify the culprit (usually Pride, Envy, Lust or Sloth) and cast them out. Whether it was truly a demon or not, identifying the source of my inner conflict helps me to make the correct decision or at least point my thoughts in the right direction.