By multiplying from the left with ( p̸ + m ) (dropping unit matrices from the notation) and using properties of the gamma matrices, p̸ p̸ = 1 2 ( p̸ p̸ + p̸ p̸ ) = 1 2 ( γ γ μ μ p μ μ γ γ ν ν p ν ν + γ γ ν ν p ν ν γ γ μ μ p μ μ ) = 1 2 ( γ γ μ μ γ γ ν ν + γ γ ν ν γ γ μ μ ) p μ μ p ν ν = g μ μ ν ν p μ μ p ν ν = p ν ν p ν ν = p 2 ,