It seems to me that we at least have to admit two different classes of proposition:

1) Propositions that reflect or imply an expectation of some experiences over others. Examples include the belief that the sky is blue, and the belief that we experience the blueness of the sky mediated by photons, eyes, nerves, and the brain itself.

2) Propositions that do not imply a prediction, but that we must believe in order to keep our model of the world simple incomprehensible. An example of this would be the belief that the photon continues to exist after taxes outside of our light cone.