[Kaidan's brain helpfully points out that he can reave. He manages to keep from saying as much, although there's another pause.]

That depends. Are your Reavers giant sentient machines on a mission to wipe out all organic life from the galaxy, apparently because somebody programmed them wrong?

[Shepard may have explained about the Catalyst and the Leviathans, but even his own last memory of staring worriedly into a grey alien sea doesn't make that last bit any easier to believe.]