and s representing the spin direction of the particle. In this formulation, according to Esposito, quantum mechanics must necessarily be interpreted in probabilistic terms, for the reason that a system's initial motion condition cannot be exactly determined. Esposito explained that "the quantum effects present in the Schrödinger equation are due to the presence of a peculiar spatial direction associated with the particle that, assuming the isotropy of space, can be identified with the spin of the particle itself". Esposito generalized it from matter particles to gauge particles, in particular photons, for which he showed that, if modelled as ψ ψ = ( E − − i B ) / 2 , with probability function ψ ψ ∗ ∗ ⋅ ⋅ ψ ψ = ( E 2 + B 2 ) / 2 , they can be understood in a quantum potential approach.