Using this expression, it is possible to solve Laplace's equation ∇ φ (x) = 0 or Poisson's equation ∇ φ (x) = − ρ (x), subject to either Neumann or Dirichlet boundary conditions. In other words, we can solve for φ (x) everywhere inside a volume where either (1) the value of φ (x) is specified on the bounding surface of the volume (Dirichlet boundary conditions), or (2) the normal derivative of φ (x) is specified on the bounding surface (Neumann boundary conditions).