# kyleherndon comments on The metaphor you want is “color blindness,” not “blind spot.”

• Color blindness is a blind spot in color space.

• This is true, but for it to be true you need to take a maybe-not-obvious view either of what “colour space” is or of what the other space in which we’re thinking about blind spots is. The following may all already have been in Kyle’s head, but it took a minute to get it into mine and I may save someone else some trouble.

So what’s not true is that colour-blindness involves being unable to see colours in some subset of an RGB-like space, in the same sort of way as a blind spot is being unable to see things in a particular subset of (x,y,z) space. Having a blind spot doesn’t cut bits out of your colour-space, it projects it onto a smaller subspace. (Approximately.)

But either of the following perspectives works.

1 Instead of “colour space”, say something like “wavelength space”. Colour-blindness is a bit like being unable to detect light in a particular range of wavelengths. You can think of the task of your visual system as reconstructing something like a function from (x,y,z,wavelength) to amount-of-light, and both blind spots and colour-blindness are something like being made unaware of a particular region of that space.

2 Instead of thinking of physical (x,y,z) space, think of the space of possible sense-data at a given moment. Positions (or, more accurately, directions out from your eye) form the basis for this space, not its elements, and what you see is a function from (x,y,z) to something-like-colour. Having a blind spot means projecting this space down into a smaller subspace (one that ignores the values of that function in certain places), just as being colour-blind means projecting something like (R,G,B) space down into a smaller subspace.

And although those are both a little non-standard, I think either of them is more faithful to how the visual system actually works than making any sort of analogy between (x,y,z) or (r,theta) and anything like (R,G,B).

(Note: Of course our actual colour space is not all that much like (R,G,B) for various reasons, and the physical thing we’re perceiving is more like (r,theta) than (x,y,z) but also we have two eyes, and colour-blindness isn’t just a matter of not detecting light in a particular range of wavelengths; the above is all deliberately sketchy.)