The Source Code plays on the brain-in-vat scenario. Characters never show Stevens his true condition. They describe his true condition, but they don't show him because that would be too depressing. Yet in principle, a brain-in-a-vat can see itself. It can't see itself using its own sensory organs. But in the Source Code model, it would be possible to point the camera at the truncated body of Stevens and input that data stream directly into his brain.
And that's analogous to our own sensory self-perception. I look at my hands with my eyes. An external stimulus feeds information into my eyes, which transmits information to the brain and consciousness. In that respect, we're like brains-in-vats, seeing ourselves via a security camera connected to a neurointerface.
But there's a catch. In Source Code, there are characters outside the vat. They can see the vat in the room. They are truly external observers.
By contrast, every human, individually and collectively, is inside the vat. We think we're are seeing ourselves as we really are, but there's no objective frame of reference. No third-person perspective.
The only individual who's truly in a position to know what we are really like is God. And God can communicate his perspective to us.
Absent propositional revelation, there's no check on idealism or solipsism. There's no way to lift the veil of perception. No way to bridge the gap between perception and reality.