Smelling matter

Abstract

While the objects of olfaction are intuitively individuated by reference to the ordinary objects from which they arise, this intuition does not accurately capture the complex nature of smells. Smells are neither ordinary three-dimensional objects, nor Platonic vapors, nor odors. Rather, smells are the molecular structures of chemical compounds within odor plumes. Molecular Structure Theory is offered as an account of smells, which can explain the nature of the external object of olfactory perception, what we experience as olfactory objects, and what determines the olfactory quality of smells by which we can demarcate the spatiotemporal boundaries of smells.

Keywords:

• Molecular Structure Theory
• object of perception
• odors
• odor theory
• olfaction
• perceptible object
• smells

Notes

1. Olfaction guides our food choices (Fallon & Rozin, 1983), dietary preferences (Rozin, 1978; Rozin, Hammer, Oster, & Marmora, 1986), our selection of mates (reviewed in Havlicek & Roberts, 2009; Young, 2014) and social acquaintances (Herz & Schooler, 2002; Jacob, McClintock, Zelano, & Ober, 2002; Li, Moallem, Paller, & Gottfried, 2007), and is responsible for identification of kin (Porter, Balog, Cernoch, & Franchi, 1986; Porter, Cernoch, & McLaughlin, 1983; Russell, 1976).

2. Plato’s theory of smell is contained within two paragraphs of the Timaeus 66d–67a, while Aristotle’s theory is developed in De Anima and De Sensu. Johansen (2006) provides a detailed assessment of the differences between Aristotle’s theory in De Anima and De Sensu. Additionally, for an introduction to the commentators’ debate over Plato and Aristotle’s theories of olfaction, see Kemp (1997).

3. The Intentional Object question is similar to giving an account of what Pylyshyn (2003, chapter 1) refers to as the “phenomenal” content of perception.

4. Since MST is offered as a refinement of OT, no general theory of perception will be assumed throughout the paper. The paper presents MST and the evidence for this new modulation of OT in the most neutral manner so as to be compatible with the variety of different theories of perception held by proponents of Odor Theory.

5. A good example of this is coffee, whose molecular components have been calculated at 150 (Clarke, 1986), while only a dozen or so of these constituents generate the aroma of coffee.

6. I am indebted to Lycan (1996) for this kind of example, though his construal of examples of this variety is far more nuanced and depends upon multiple levels of representation (Lycan, 2014), since the olfactory object considered as an odor on his view might not be considered an ordinary object. Nonetheless, the phenomenal quality of smell might be correct, thus making it a case of perceptual misrepresentation, but not a misrepresentation of the smell experience.

7. Given that smells do not present entities with surfaces and clearly delineated boundaries their ability to maintain a recognizable mereologically complex structure is the closest approximation of the criteria Spelke (1990) sets out for visual object recognition.

8. Batty (2010a, 2010c) dismisses these experimental results on the grounds that they are atypical of olfactory experience, perhaps because she assumes the correctness of the common sense conception of the phenomenology of olfaction. However, further argumentation is required as to why our phenomenology of olfaction should be considered authoritative and how her conception of the “typicality’” of an olfactory experience is determined.

9. A perfume’s sillage is an excellent example of the spatial aspect of an olfactory object. In designing a new product, perfume chemists must consider a perfume’s sillage—the diffusion rate across space of a perfume. Some scents are designed to announce their wearer’s presence or to turn the heads of those in a room, while colognes are designed to be noticeable only within a small radius around the wearer.

10. Mizobuchi et al. (1999) have documented a case of olfactory hallucination in which a patient reported hallucinations of a bad smell that always presented itself as coming from a particular direction, suggesting that olfactory experience has a spatial dimension wherein smells can be identified.

11. A fuller discussion of these claims can be found in Young (2014), with particular emphasis on how widening what is considered olfactory perception by adding trigeminal stimulation yields synchronic experiences of odor locatedness.

12. It might be suggested that synthetic smells should be treated as non-veridical perception. However, given that the external object of olfaction is considered to be a gaseous chemical cloud, synthetic chemical clouds would be veridically perceived. This line of reply would then require that though the perception of the external object is veridical the experience of its olfactory quality is not. However, further argumentation is required to explain why we should consider it possible for the objectual perception to be veridical, while the experience of its quality is not.

13. In keeping with contemporary chemoscientifc practices, olfactory perception in humans is specified and limited to the olfactory sensory system and its proper sensible, where the later is the issue at hand. The olfactory sense organ starts from the olfactory epithelium, including the mucus layer, extending through cortical processing within the piriform cortex and olfactory cortex (for a short introduction to the olfactory system see Bensafi et al., 2004; Young, 2011, chapter 1). Sensory motor sensations of breathing, sniffing, and the tactile stimulation of the nasal cavity will be excluded from consideration, as well as trigeminal stimulation. Though trigeminal stimulation influences the phenomenal content of reported smell experience, the trigeminal system has its own sensory qualities (Filiou et al., 2015). Thus, instances of trigeminal stimulation influencing olfactory quality should be considered as cross modal effects.

14. The perceptible boundaries of chemical compounds yielding an odor are very sharp, as demonstrated by Wrobel and Wannagat (1982). Using benzenoid musk, they showed that replacing one of its carbon atoms with a larger silicon atom causes the entire molecule to become odorless.

15. Tempting as it might be to claim that all matter smells, the less ambitious epistemologically safe route should be embraced. Every chemical compound that has a structure falling within the range of being biologically detectible will have the perceptible property of smelling. Whether or not other species have access to a greater number of smells or smell things differently than us should be bracketed, as these issues should be settled in accordance with the range of molecular structures they can transduce using their olfactory system. Tentatively, the more similar the transduction mechanisms between species, the more likely the same or extremely similar olfactory qualities are to be perceived.

16. Since the olfactory system is ontogenetically ancient, a great deal can be inferred about human olfactory experiences from animal models. Aside from the fact that it is a generally accepted practice throughout the sciences, Aristotle (DA II 7419a33–b1; DA II.9, 421a8–422a8; De Sensu 5, 422b27–445b2) also supports the methodology of thinking of olfaction as lying on a continuum with other organisms. Animal models do not provide any further evidence for the claim that olfactory objects are the structures of molecular compounds, but only reinforce it (e.g., both rodents and aphids can discriminate between enantiomers; Glinwood, Du, & Powell, 1999). Thus, for the sake of brevity, evidence from animal models has been omitted.

17. The phenomenon of interpersonal variability in an odor’s olfactory quality, whereby two individuals report different olfactory qualities of the same stimuli can be explained in light of odotope theories, which claim that smell (OQ) is determined by a synthetic process across multiple olfactory receptor neurons (ORNs). Given the genetic variance in determining the number, kind, and distribution of ORNs, the intersubjective variance is attributable to differences at the receptor level in sensitivity to different molecular structures (Keller et al., 2007; Mainland et al., 2014).

18. The most exhaustive list of the smells of enantiomers is maintained by John Leffingwell. According to Leffingwell’s calculations listed in Turin (2006), 64% of enantiomers smell the same, 17% smell different, while the smell of the remaining 19% is currently unknown.

19. A more detailed discussion of the representational structure of olfactory mixtures and their configural and elemental nature can be found in Young (2015).