This paper begins by outlining Hume's understanding of perception according to which ideas are copies of impressions, which are thought to constitute a foundational confrontation with reality. This understanding is contrasted with Peirce's theory of perception according to which percepts give rise to perceptual judgements, but perceptual judgements are not a copy but an index (or 'true symptom' - just as a weather-cock indicates the direction of the wind) of the percept. Percept and perceptual judgement are thereby able to mutually inform and correct one another in rich ways, as the perceiver develops mental habits of interpreting their surroundings.