For illustration, consider a physical system of fields that behaves the same under translations in time and space, as considered above; in other words, L ( φ φ , ∂ ∂ μ μ φ φ , x μ μ ) is constant in its third argument. In that case, N = 4, one for each dimension of space and time. An infinitesimal translation in space, x μ μ ↦ ↦ x μ μ + ε ε r δ δ r μ μ (with δ δ denoting the Kronecker delta), affects the fields as φ φ ( x μ μ ) ↦ ↦ φ φ ( x μ μ − − ε ε r δ δ r μ μ ) : that is, relabelling the coordinates is equivalent to leaving the coordinates in place while translating the field itself, which in turn is equivalent to transforming the field by replacing its value at each point x μ μ with the value at the point x μ μ − − ε ε X μ μ "behind" it which would be mapped onto x μ μ by the infinitesimal displacement under consideration. Since this is infinitesimal, we may write this transformation as