Very, very natural for us to display some degree of fear in certain situations. So for example, if you're standing at the top of a high building, it's probably understandable if you feel some degree of fear, because it's a situation which represents some level of danger. A phobia is when that fear becomes all-consuming or irrational. So for example, if we have a fear of spiders and we are constantly looking around to see if there are spiders in the room. That would be an inappropriate level of fear and would be in the realms of phobia.