In Feynman lectures on gravitation section 1.4, he tries to debate over whether one should quantize the gravitation or not. He provides a two-slit diffraction experiment with a gravity detector, which is assumed to be classical. The position of electron is described by amplitude, and it cannot be that a particle described by an amplitude has an interaction (gravitation here) described by probabilities. He then claims that if the gravity interacts through a field, it follows that the gravity field must have an amplitude corresponding to gravity field of an electron pass through upper slit and the other. It is confusing to use 'amplitude' and 'probabilities' to separate the concept of 'quantum' and 'classical'. As far as I'm concerned, Feynman's 'amplitude' means the wave function of the particle, on the other hand, the probabilities means that the observer does several measurements for independent experiments, and record the probabilities of an electron passing through the upper slit and passing through the lower slit.
I'm not sure about my opinion on Feynman's terms is correct?
And he points out in page 15, that the amplitudes would become probabilities for very complex objects, which then confuses me, can anybody explain clearly what he means here?