All ideas are traceable to prior sense impressions

Hume opens by insisting that every idea in the mind, however complex or fantastical, can ultimately be broken down into simpler components that trace back to something once experienced through the senses or felt internally. A unicorn is just a horse-impression fused with a horn-impression; nothing in the mind is created from nothing.

This sounds modest but does real philosophical work: it becomes Hume's tool for testing whether a supposedly meaningful concept — like an immaterial soul, or a necessary connection between cause and effect — actually corresponds to any experience at all, or is merely a word we've grown comfortable using without content behind it.

He proposes this as a kind of intellectual hygiene: whenever a term seems suspicious or overused in philosophy, trace it back to the impression it supposedly derives from. If none can be found, treat the term with suspicion rather than reverence.

Takeaway: before accepting an abstract concept, ask what concrete experience it's actually built from.