The idea of necessary connection is derived from an 'impression of reflection' (an internal feeling or sentiment), not from an 'impression of sensation' (an observed instance before the mind)
An internal feeling or sentiment, as opposed to an externally observed instance
impression of sensation(Hume's epistemological distinction contrasted with impression of reflection)
An observed instance before the mind, i.e., a direct sensory experience
necessary connection(Hume's account of the origin of the concept of causation, Treatise I.III.14)
The felt sense of the mind being pulled from one impression to an associated idea, which the mind then projects onto external objects as if it were a connection among the objects themselves
Thus, the custom or habit to make the inductive inference not only gives rise to a new idea of not yet observed instances resembling the instances we have already observed, it also produces a feeling of determination to make the very inductive inference in question. This feeling of determination, in turn, gives rise to a further new idea, the idea of necessary connexion, which has no resemblance whatsoever with anything we have observed. It is derived from an “impression of reflection” (an inter