Let’s play a little game, hopefully illustrating something about quantum field theory, quantum gravity, and all that other good stuff. We already know that particles can be viewed as excitations of fields, which are some continuous media filling up space. One way of illustrating this is by drawing a checkers board filled with black or white squares (on second thought, most of what is below might be more intuitive to the go player). We say that the square is white when the field is not excited, and black when it is excited at some location. In other words, it costs a fixed amount of energy to paint one of the squares black (actually these turn out to be dark brown, which is some kind of sad statement about my eyesight).
Obviously in the lowest energy state all the squares on the board are white, and the excitation costing least energy is obtained by painting one square black, and that could be anywhere. This is what we can call a single particle:
Shores of the Dirac Sea 