Hi, im having trouble understanding these concepts. I checked out some websites but it still doesnt help. First of all what's the main postulate? That there exist 2 different kind of particles: bosons and fermions? What are their fundamental definitions which lead to the fact that an integer number of fermions can occupy ther same energy level, only 0 or 1 bosons can occupy same energy level. In what situations does this apply? Must it be a closed system or a system of particles so close together that the heizenberg uncertainty priciple dominates? Also what is meant by "the same energy" level (same numerical value of energy?) (because im sure there's one fermion here with energy = a and if you look thousands of km away youll find another one with the same energy...) Im sure this is an easy question for you guys..