It's NOT hard to fool a nose

People, I've decided, are weird. Common sense tells us that reality is objective, as anyone who's ever been hit in the head with a brick will attest to (as long as the brick didn't hit them too hard, at least). Common sense would further suggest that determining what that objective world is is sometimes quite difficult, but possible with sufficient effort - repeat testing, multiple witnesses, that kind of thing.

Unfortunately there's no way avoid the slippery slope that will hurl the inquiring mind towards the unavoidable Big Question, "what do we mean by objective reality, anyway ?". Luckily, this is quite different - fundamentally different - from the question of what objective reality really is ("what's it all about, when you get right down to it ?"). What we mean by objective reality is, I think, simple enough : it's what gives rise to our perceptions. Ideas like "reality is nothing like what we think it is", which are oddly common, are thus rendered impotent. We can't ever know "the truth" about reality, we just know that it's what gives rise to what we see. That's all we can ever define it as. Trying to describe it on some silly deeper level is pointless.

To recall an earlier example : a fish is that which always gives rise to what we perceive as a fish. Asking what the true, Platonic nature of a fish really is, or saying that a fish is probably nothing like what we perceive it to be, is just being a bit daft.

What does this have to do with the nose ? Well, the author of this Aeon piece points out that the nose is much neglected in studies of human cognition and philosophy. His main point, which is well made, is that the nose works in a fundamentally different way to the eye. It's not as poor a sense as you might think :

Humans can sense the presence of particular odorants (smelly molecules) in dilutions of less than one part in several billion parts of air... The reason why you are seldom aware of this chemical blast of information around you is straightforward: your nose is actually a bit too good at its job for your brain to handle. If your mind was consciously processing all the molecular information that your nose picks up, without any break in its conscious awareness, you’d soon agree to a lobotomy just to ease your mind.

But the eye is capable of detecting just a single photon, and again the brain does some highly complex signal processing to stop us being overwhelmed the whole time. And one photon is not one among mere billions, but one in ~10²⁰. Or more. So vision wins there, hands down.

Where things get more complicated is when it comes to subjectivity versus objectivity. I agree (of course !) that smells are generated objectively, but I don't think there's any need for a defence of the objectivity of the nose : who thinks smell is especially subjective ? No-one I know. It's a bit straw-man-ish. Yes, we smell the same thing in different ways, and can like or dislike them differently, but that's no different from any other sense.

When you smell a mix of isovaleric acid, for example, it makes a difference if I tell you that what you perceive is the odour of Parmesan or vomit... On its own, indole has a strong and characteristic faecal smell. Yet you won’t perceive it in the overall coffee aroma – unless you are a pregnant woman. Pregnancy alters your sensitivity toward specific components that can act as potentially harmful contaminants... Consider androstenone, a pig pheromone, that people perceive quite differently. To some, it smells like urine, and to others like body odour (often noted as unpleasant). Others find androstenone has a woody and even a vanilla or floral note... Perceptual reactions to smelly molecules are variable not because they represent subjective whims but because small changes in the chemical composition of the environment are what the olfactory system evolved to evaluate, objectively – and pretty reliably so.

This section seems to undermine its own main point. Yes, my eyes can be fooled too, but they generally give me a reliable view of the world. If I see I'm about to walk into a tree, then unless that tree is actually a very clever and carefully-created illusion, then my eye has given me a useful and correct perception of reality. The nose seems to be much more limited, since it's tied so directly to other senses and even our higher reasoning. I can't use my nose to reliably determine much of anything, since context is so essential. In contrast, vision on its own appears to me to be very much more useful 99% of the time.

(Yes, one could imagine some clever setups to reverse this, like making a disgusting food look appetising but retain a horrible odour. But this would be rare outside of a lab.)

These processing differences explain why olfaction affords greater variation in the perceptual interpretation of the physical stimulus. One might wonder how human olfaction could provide us with objective access to reality despite such prevalent variation in perception. The alternative is to suggest that it provides us with access to reality because of such variation. Indeed, it might be easier to fool your eyes than your nose!

This bit doesn't make sense to me. If I see a picture of a deer and tell someone it's an elephant, they'll say, "no it's not." But I I tell them they're smelling vomit when they're really smelling parmesan cheese, and I can fool them by this simple trick, then that seems to make a nonsense of the idea that the nose is hard to fool. The nasal sense provides much less direct access to reality than the others.

Certainly, olfaction is much more variable in its perceptual expression than vision. But this variation doesn’t mean that the nose has been misled. Olfaction simply does not work analogously to the visual system. Crucially, feature coding in smell is not viewpoint-invariant. On the contrary, the causal principles of the olfactory system facilitate a cue-dependent interpretation in the computational integration of its neural signals. 

Okay, a fair point that the nose is supposed to work in tandem with other senses, but I don't see how you can say it hasn't been misled if it gives you a smell of vomit when you're actually smelling a delicious cheese.

Why assume that perspective invariance is the criterion of objectivity? What characterises objectivity as the hallmark of reality is not the appearance of things, which is changeable. Instead, we need to understand the underlying causal principles that give rise to this appearance and its variability. Smell appears fickle only without a sufficient understanding of its causal mechanisms.

Whut ? Look, if smell gives me the wrong answer more than vision does, then I say that it's a less useful and less accurate sense than vision. The criteria for objectivity is to establish how what we sense will influence our other senses in other conditions. The most objective sense is the one that gets to the truth - this perspective-invariant thing that we can use to infer our perceptions in other circumstances - with the greatest precision and accuracy. Start defining objectivity by anything other than perspective invariance and you end up with a strange world indeed, and the author's own examples show quite convincingly that smell just isn't as good as that (at least in humans) as vision is.

Why might it be easier to fool your eyes than your nose? - Ann-Sophie Barwich | Aeon Essays

Your nose is the best biosensor on the face of the Earth. This claim must sound counterintuitive since the sense of smell has acquired a rather poor reputation over the past centuries. Philosophers and scientists alike have only rarely singled it out for close study.