Category: epistemology
Type: thought experiment
Associated thinker(s): David Hume
Original publication or approximate period: An Enquiry concerning Human Understanding (1748), Section II, §8
Difficulty: Intermediate
Themes: empiricism, ideas and impressions, imagination
Related experiments: Mary’s Room: The Knowledge Argument; Molyneux’s Problem
Hume’s copy principle says that simple ideas in their first appearance derive from corresponding simple impressions. His account includes impressions of sensation and reflection, and allows the mind to combine and rearrange experienced materials. In Section II, §8 of the Enquiry, he considers a person who has seen every shade of blue except one. The shades are laid out in order from darkest to lightest, with a gap where the missing shade belongs.
Hume suggests that the person may notice the gap and imagine the absent shade without having seen that precise shade before.
Can we form a genuinely new simple idea without a matching impression, and what would that show about Hume’s copy principle?
The case appears to be an exception to the rule that each simple idea is copied from a corresponding impression. Hume concedes that simple ideas are not invariably derived from matching impressions, but calls this case so singular that it does not warrant changing his general maxim.
The comparison with Mary’s Room is a later teaching connection, not part of Hume’s argument. The missing-shade passage is about Hume’s account of ideas and impressions; it does not settle what Mary learns in a different thought experiment.
Hume’s own stated response is that the case is singular and does not require changing the maxim. The Stanford Encyclopedia of Philosophy discusses a later interpretation, associated there with work by Fogelin, Garrett, and Durland, that the unusual gradation makes the case too limited to undermine the principle’s broader use. That is an interpretation of Hume’s response, not a reason Hume gives in the passage or a settled consensus. The primary text does not explain the missing shade as a complex idea composed from neighbouring shades.
Hume does not say that this exception refutes his general principle, and the case is not a general proof that knowledge can arise without experience. It concerns filling a gap between familiar shades, not imagining a colour entirely unlike any colour one has encountered.
As teaching extensions, one can ask whether a person could imagine an unfamiliar sound or taste, where there may be no neighbouring impressions to interpolate between. These are not examples supplied by Hume; they test whether the same reasoning applies beyond the gradation of blue shades.
The relevant passages in Hume’s Enquiry and Treatise were consulted directly, as was the cited SEP discussion. The missing-shade example appears in Enquiry II.8 (1748); the Treatise contains an earlier formulation of the copy principle, not the example. Connections to Mary’s Room and other cases are presented as teaching comparisons. This is an original educational explanation, not a substitute for the texts or a survey of the full scholarship.
Research check: 2026-11-01. Claim-level evidence and access limits are recorded in the source-review ledger.