(NC) “We never observe any such causal power in any of our experiences” and see where (NC) fits into his attack and also give a better explanation of what (NC) means.
Hume starts his justification of (NC) by stating that every idea we hold is inspired from an impression in the world. Thus, the concept of a circle comes from circular objects we observe in the world with our senses. It is this sensory experiences that allows humans to formulate the idea of a circle and similarly all other ideas. It seems he is making the unstated claim that only experience can be the root