This is a question I've had for some time now. Why is the exchange interaction not considered a force, like the other 4 fundamental forces? When reading solid-state physics texts, for example, I come across explanations of this kind: the atoms cannot get too close together because of the exchange interaction of the electrons. I guess an answer could be that there is no carrier particle supplying the force, such as the photon. But to me this is an unsatisfactory answer because why can't we simply expand our definition of force to interactions where no carriers are necessary? All we should care about is the net result on the particles experiencing the interaction, and in that sense I don't see why the exclusion principle cannot be considered a force. So, why is the exchange interaction not a force? What part of the interaction is different from among the other four forces that prevents it from being considered one? A side-question: When you seat on a chair, my electrons are repeled by the electrons in the chair, so that I don't fall through. Is this electrical repulsion the only cause for me not falling through, or is it also the Pauli exclusion principle? Or is it ONLY the Pauli exclusion principle? I find these things confusing, I hope someone can help me clear this up.