  We investigate the three-dimensional, time-reversal invariant topological
superconductors with generic interaction by their response to external fields.
The first description is a gravitational topological field theory, which gives
a $Z_2$ classification of topological superconductors, and predicts a
half-quantized thermal Hall effect on the surface. The second description
introduces an s-wave proximity pairing field on the surface, and the associated
topological defects give an integer $Z$ classification of the topological
superconductors.
