The Pauli exclusion principle refers to the fact that certain particles cannot be at the same place at the same time, with the same energy. Only fermions (examples are protons, neutrons and electrons) are bound by the Pauli exclusion principle, while bosons (an example is a photon - light beam) are not. A more precise way to describe the Pauli exclusion principle is to say that two of the same kind of fermions that are in the same quantum system (same atom, for example) cannot have the same quantum numbers. This principle was discovered by physicist Wolfgang Pauli in 1925. It is a very important principle in physics because the particles that make up ordinary matter are fermions.