A small part of my brain has been bugged by this for a while now, so I figured I'd ask. According to most teachings, as well as the wikipedia entry, there are only 4 fundamental forces. The strong and weak interaction, electromagnetism and gravity, and as far as I know the Pauli exclusion principle (PEP) has nothing to do with either one of those. What's been bugging me is that for all practicle purposes the PEP seems to act like a force. For example, on the wiki page on forces it list the four fundamental forces, and then proceeds to list a number of forces that can all be dervied from these, but on several occasions throughout the text, e.g. while explaining the normal force, they say that it comes from the PEP, with no further link back to any of the four fundamental forces. Further, in the case of a neutron star the situation is even more telling, namely that the size of the star is given simply by the relation of the gravity pressing matter inwards, and the fermi pressure, casused directly by the PEP, pressing matter outwards. This makes it seem very much like a force to me. So, question: Why is the Pauli exclusion principle not the fifth fundamental force?