The Missing Shade of Blue—critical observations.

It is surprising that Hume offers a counter example almost immediately after saying that this is all that is required to refute his claim. Some scholars have said that this is precisely the kind of example that should have led Hume to reject his copy principle. A more generous interpretation is that Hume is knowingly modifying his general claim and it should now be understood as saying, ‘In the normal course of events...’

A counter example isn’t necessarily more damaging to Hume’s epistemology than a counter example is to Newton’s laws of motion—his epistemology might be perfectly adequate within certain limits.

Throughout this section Hume assumes that colours are simple impressions and simple ideas. This can be challenged and if a colour can be complex due to it being possible to analyse it down into hue, tint, tone and shade, or even being able to construct them from a mixture of red, green and blue, then the ability to imagine new colours becomes less problematic.

If our perception of a colour depends not just on the colour itself but also on the context within which that colour is seen then it is no longer possible to conclude that just because two adjacent colours are indistinguishable the ends of the gradient will be indistinguishable.

For the reductio argument to work you would have to assume that indistinguishable impressions and the corresponding indistinguishable ideas were, nevertheless, different. It isn’t clear that there can be ideas that are both indistinguishable and different.

Hume is also assuming that our idea of a gradient is the same as perceiving a series of graduated steps and this isn’t necessarily so.

This isn’t obviously true if the steps in the gradient are very small. The difference between steps may be so small that not only are adjoining steps indistinguishable but when one is removed the newly adjoining steps remain indistinguishable. Again, this may not be fatal to the thought experiment. It would be possible to specify a sequence of steps such that normally experienced people can perceive a gap and make the claim that the experience deficient person would also recognise the gap.

Clearly Hume is using blue as just an example and it is important to avoid the naive criticism that Hume is unaware that there might be a missing shade of green or a missing note on a scale, etc. The singularity presumably refers to the type of example, i.e. the tightly ordered sequences. Perhaps the challenge that Hume should have offered is, ‘If anyone thinks that this kind of example does undermine the claim that in the normal course of events...then all they need to do is explain why that is so.'

Hume presents this as a counter example to his copy principle, i.e. that every simple idea is a copy of the corresponding simple impression.

Each individual colour, even those that are similar, generate corresponding individual ideas.

This is also true of individual shades of the same colour.

This can be proved by a reductio ad absurdum — if the individual colour ideas were not different you could prove that two very different colour ideas were the same.

Choose blue as an example

Imagine someone who has seen all colours except a particular shade of blue.

Present them with a gradient of all the shades of blue except the one they have never previously encountered.

Claim 1: they will notice that there is a missing shade and 'a greater distance in that place between the contiguous colours than in any other.'

Claim 2: that from their own imagination they will be able to generate an idea of the missing shade even though they had never had the relevant impression.

Hume’s evaluation: This is a ‘singular’ example and doesn’t undermine the general rule.