The simple formula [ x , p ] = i ℏ ℏ I , valid for the quantization of the simplest classical system, can be generalized to the case of an arbitrary Lagrangian L . We identify canonical coordinates (such as x in the example above, or a field Φ(x) in the case of quantum field theory) and canonical momenta π x (in the example above it is p, or more generally, some functions involving the derivatives of the canonical coordinates with respect to time): π π i = d e f ∂ ∂ L ∂ ∂ ( ∂ ∂ x i / ∂ ∂ t ) .