  We present a derivation of the scattering amplitude prescription for the pure
spinor superstring from first principles, both in the minimal and non-minimal
formulations, and show that they are equivalent. This is achieved by first
coupling the worldsheet action to topological gravity and then proceeding to
BRST quantize this system. Our analysis includes the introduction of constant
ghosts and associated auxiliary fields needed to gauge fix symmetries
associated with zero modes. All fields introduced in the process of
quantization can be integrated out explicitly, resulting in the prescriptions
for computing scattering amplitudes that have appeared previously in the
literature. The zero mode insertions in the path integral follow from the
integration over the constant auxiliary fields.
