Hello guys, here's something that's been bothering me for a while now: We know the Pauli exclusion principle states that 2 Fermions cannot be in an identical state. So then we have systems like solids or free electron gases, and we calculate and form Fermi-Surfaces, based on the fact that the electrons are being distributed to different states. What I don't understand is - what defines a "system" in which this principle holds? We could have another gas next to our gas, and the electrons would once again take the states from ground state up. So how does a physical system "know" its a system, such that two electrons in it cannot have the same states? What if I have two "distinct" electron gasses and suddenly i instantaneously mix them? Would then half of the electron take hold of higher energy states in an instant? I hope the question isn't vague. Thanks a lot! Tomer.