David Hume is one of the most famous defenders of empiricism – the view that all knowledge ultimately comes from experience. In Treatise of Human Nature (1739) and later Enquiry Concerning Human Understanding (1748), Hume argues that the human mind is built entirely from two types of experience: impressions and ideas. But there’s a famous passage in Hume’s work that seems to undermine his own argument: the missing shade of blue.
Impressions and ideas
Hume’s empiricist epistemology distinguishes between two types of experience – impressions and ideas:
- Impressions are the actual perceptions we have when we see, hear, or touch things.
- (E.g. the sight of a bright red apple or the feeling of warmth from a fire.)
- Ideas, on the other hand, are the faint images of these impressions in our minds when thinking about or remembering or reasoning about these impressions.
- (E.g. remembering that red apple or imagining a fire.)
“All the perceptions of the human mind resolve themselves into two distinct kinds, which I shall call IMPRESSIONS and IDEAS… The full examination of this question is the subject of the present treatise; and therefore we shall here content ourselves with establishing one general proposition, THAT ALL OUR SIMPLE IDEAS IN THEIR FIRST APPEARANCE ARE DERIVED FROM SIMPLE IMPRESSIONS, WHICH ARE CORRESPONDENT TO THEM, AND WHICH THEY EXACTLY REPRESENT.”
– Hume, Treatise of Human Nature, Book 1 Part 1
This is known as the copy principle: according to Hume, all ideas can be traced back to some original sensory impression. They are copies that exist in the mind of these original impressions.
For example, you can have an idea of golden mountain because you’ve had impressions of gold and mountains, even if you’ve never seen the two combined.
But you couldn’t have an idea of a new colour that you’ve never seen before because there’s no impression of it to begin with.
Hume also distinguishes between simple and complex ideas. Simple ideas are the most basic components of thought and can’t be broken down any further – for example, the specific shade of blue you see in the sky. Complex ideas, in contrast, are combinations or arrangements of these simpler ones – like imagining a blue door, which combines the simpler ideas of ‘blue’ and ‘door.’
Hume’s point is that even our most complex thoughts can ultimately be broken down into simple ideas, and every simple idea can be traced back to a corresponding impression.
The missing shade of blue
However, Hume raises an objection to his own theory:
“There is, however, one contradictory phenomenon, which may prove that it is not absolutely impossible for ideas to arise, independent of their correspondent impressions. I believe it will readily be allowed, that the several distinct ideas of colour, which enter by the eye… are really different from each other; though, at the same time, resembling. Now if this be true of different colours, it must be no less so of the different shades of the same colour; and each shade produces a distinct idea, independent of the rest. For if this should be denied, it is possible, by the continual gradation of shades, to run a colour insensibly into what is most remote from it; and if you will not allow any of the means to be different, you cannot, without absurdity, deny the extremes to be the same. Suppose, therefore, a person to have enjoyed his sight for thirty years, and to have become perfectly acquainted with colours of all kinds except one particular shade of blue, for instance, which it never has been his fortune to meet with. Let all the different shades of that colour, except that single one, be placed before him, descending gradually from the deepest to the lightest; it is plain that he will perceive a blank, where that shade is wanting, and will be sensible that there is a greater distance in that place between the contiguous colours than in any other. Now I ask, whether it be possible for him, from his own imagination, to supply this deficiency, and raise up to himself the idea of that particular shade, though it had never been conveyed to him by his senses? I believe there are few but will be of opinion that he can: and this may serve as a proof that the simple ideas are not always, in every instance, derived from the correspondent impressions;
– Hume, Enquiry Concerning Human Understanding, §2 (emphasis added)
Hume imagines a person who has seen every shade of blue except one – say, a particular shade between two others on a colour gradient. If you showed them this colour gradient, would they be able to imagine the missing shade of blue in the middle despite never having seen it?
Hume’s answer is yes – they would be able to imagine the missing shade of blue.
But this contradicts his earlier claim that every idea must be derived from a prior impression. The copy principle said every idea has a corresponding impression. But there’s no copy in the case of the missing shade of blue – no corresponding impression – it’s just a lone idea. So this undermines the claim that all ideas are just copies derived from experience.
So does this example undermine his theory?
Hume’s response to the missing shade of blue counterexample is basically just to dismiss it. He says:
“This may serve as a proof that the simple ideas are not always derived from the correspondent impressions; though this instance is so singular, that it is scarcely worth our observing.”
– Hume, Enquiry Concerning Human Understanding, §2
And that’s the end of the matter as far as Hume is concerned – that it’s “scarcely worth our observing”.
But philosophers have taken the missing shade of blue example seriously ever since because it raises a serious challenge for empiricism. If we can imagine a colour we’ve never seen before, doesn’t that prove the mind can generate new ideas and not just copy them? And if we can generate the idea of a new colour, can the mind generate other ideas too – like moral concepts, or mathematical ones?
If the answer to these questions is yes, then it weakens Hume’s empiricism. It supports the rationalist view that some ideas and concepts do not come from experience.
Summary
- Empiricism says all knowledge and concepts come from experience, rationalism disagrees.
- Hume was an empiricist – his ‘copy principle’ says all knowledge and concepts come from two types of experience:
- Impressions (direct experiences).
- Ideas (faint copies of those experiences in the mind).
- However, the missing shade of blue presents a problem: it seems possible for someone to imagine a shade they’ve never experienced. This would be an idea without an impression – precisely what Hume said couldn’t exist.
- Hume rejects the missing shade of blue counterexample as an insignificant anomaly.
- But rationalists may insist that the missing shade of blue proves their side is correct and that empiricism is wrong: if the mind can create even one genuinely new idea on its own, then experience isn’t the whole story after all.