Ackerman asserts that smell is the most memorable sense, capable of triggering powerful, complex memories that have been dormant for years. Smells can evoke vivid visions of the past. Although we can detect more than 10,000 different odors and can even subconsciously determine a person’s gender from the scent of their clothing, smell is the “mute sense”—the one for which we have relatively little direct vocabulary. Unlike sight or hearing, smell is continuous and essential; we smell with every breath.

Ackerman explains that smell is difficult to describe because the physiological links between the brain’s smell and language centers are weak, while the connection between smell and memory is direct and powerful. When we try to describe scents, we resort to comparisons (“smoky,” “fruity”) or emotions (“delightful,” “sickening”). Her mother could only describe orange blossoms as wonderful. Because smells cannot be easily named, they retain a unique mystery and power.

Using violets as an example, Ackerman explains olfaction’s chemistry. Violets contain ionone, which temporarily blocks smell receptors so the scent seems to appear and disappear—a “flirtatious” quality favored by Napoleon and Josephine. Smell is the most direct sense: Odor molecules reach receptors in the nose and signal the limbic system directly, bypassing interpretation by language or conscious thought. This unfiltered connection triggers powerful nostalgic responses. There is almost no short-term memory for smells; recall is always long-term.

All smells fall into basic categories like floral, musky, or foul. Perfumers combine these elements to create complex fragrances such as Chanel No. 5, one of the first based on a synthetic aldehyde. Perfumes have top, middle, and base notes, with lingering base notes often derived from animal secretions like musk and civet, whose chemical shape resembles certain human steroids. To be smelled, a substance must be volatile. Much of what we perceive as “flavor” is smell, since the tongue detects only sweet, sour, salt, and bitter. The leading “stereochemical” theory of smell, devised by researcher J. E. Amoore, posits that receptors in the nose are shaped to match the shapes of smell-producing molecules, just as keys are fabricated to fit locks.

Ackerman surveys writers attuned to smell: Most famously, in his sprawling novel Remembrance of Things Past, French novelist Marcel Proust uses the smell and taste of a madeleine cookie to evoke reminiscences that fill hundreds of pages. Ackerman argues that a writer’s ability to describe smell tests their sensory and literary depth.

Ackerman shares a personal memory of tagging monarch butterflies in California’s eucalyptus groves. The mentholated scent of eucalyptus leaves transported her to childhood, when her mother used Vicks VapoRub to soothe her cough. This created a new layered memory: Eucalyptus now evokes both this childhood memory and her more recent memory of the serene butterfly grove. Later, smelling fresh eucalyptus in Manhattan brought tears to her eyes, illustrating how smell creates evolving layers of personal meaning.

Our sense of smell is an evolutionary relic from aquatic ancestors. Odors must dissolve in the watery mucus of our nasal passages. Smell was the first sense to evolve, guiding early ancestors toward food and mates. Olfactory tissue developed into the cerebral hemispheres, leading Ackerman to suggest that humans developed the ability to think because they first smelled.

While body odor may have evolved as a defense to repel predators, it has developed into a complex social signifier with meanings that vary across different human communities. Body odor itself varies by diet, genetics, and number of apocrine glands, concentrated in armpits and genitals. Ackerman states that some people of Asian descent have fewer apocrine glands than white people have and thus some find white people ripe-smelling; this, she argues, is why perfumes in Asian cultures are often used to scent rooms rather than bodies. In many cultures, the word for “kiss” also means “smell.” Tribal greetings like rubbing noses or wiping a hand in another’s armpit are rooted in scent exchange.

Every individual has a unique scent signature, influenced by diet, health, mood, and occupation. Helen Keller claimed she could identify a person’s work by smells clinging to their clothes. Western cultures often mask natural odors with perfume, but this is not universal—the Masai people of Kenya and Tanzania dress their hair with cow dung. Historically, scents have played key roles in hygiene, from herbs strewn on medieval floors to prevent typhus to perfume recipes claiming healing and even magical properties.

Pheromones are chemical signals triggering social responses like courtship or ovulation. Women living in close proximity often find that their menstrual cycles become synchronized—the McClintock Effect, after psychologist Martha McClintock; George Preti’s sweat experiments suggest that a sweat pheromone drives this phenomenon. Though no specific human pheromones have yet been identified, Ackerman notes that pheromones are ubiquitous among mammals—just as birds use song to communicate, mammals use odors and pheromones. Nonetheless, Ackerman is skeptical of commercial perfumes claiming to mimic human pheromones, noting that human behaviors are too complex to be mechanistically driven by pheromones like those of many other mammals. The psychological effect of wearing perfume likely outweighs any chemical attractant.

As early humans stood upright, vision and hearing became more critical than smell. Four-legged animals with noses near the ground have keener smell—a sheepdog has 220 million olfactory cells to a human’s 5 million. Yet human olfaction remains detailed. The human nose juts out, requiring active sniffing to draw odor molecules deep into the nasal passages, forcing humans to use their sense of smell more deliberately than many quadrupeds do.

A sneeze is a reflexive expulsion of air. Most animals sneeze through their noses, but humans sneeze with open mouths because the nasal passage has a sharp hairpin turn—an evolutionary compromise required for the development of a large brain and stereo vision. A sneeze blasts air at nearly 85% the speed of sound, scouring bacteria from nasal passages. Ackerman suggests that the noses of modern, urban-dwelling people must deal with a far greater array of sneeze-inducing irritants than they evolved to deal with.

Deer visiting Ackerman’s yard flee her human scent, leading her to consider smell as camouflage. She cites a Field and Stream recipe for a scent-masking spray made by soaking pine or mushrooms in vodka. Applying this buries human odor and fools even sensitive animal noses.

Ackerman battles deer to protect her roses, but since she loves the deer and does not want to harm them, she wards them off with smells. After foul tobacco proves effective but unpleasant, she plants lavender as an “olfactory fence.” Roses have intoxicated humans more than any flower, with ceremonial uses dating back to ancient Persia and Damascus. Ancient Romans used roses in pillows, fountains, and banquets; at one, a guest smothered under Nero’s rose petals. In Islamic cultures, the rose is spiritual, used in cooking and hospitality. In the 19th century, European horticulturalists cross-bred European roses with tea roses from China, inaugurating a practice of hybridization that has since resulted in hundreds of varieties, including some named for presidents like Abraham Lincoln and John F. Kennedy, and others named for celebrities like Dolly Parton.

Though smell is no longer essential, we remain attached to it—Helen Keller called it “the fallen angel.” Chemists restore what evolution phased out. Today, 80% of perfume industry revenue comes from scenting everyday objects like toilet paper and clothing. Malls add pizza or cake aromas to ventilation. This shows a need to create comforting “nests” of familiar scents.

Anosmia—the inability to smell—can be destructive. A gourmet who lost smell after head injury suffered depression, food poisoning, and couldn’t detect a fire. Those experiencing anosmia lose the pleasures of food, fragrance, and emotional associations. Causes include allergies, trauma, aging, or toxins. For some, sex drive disappears with the loss of smell. Women appear more attuned to partners’ scents and consistently score higher on smell sensitivity tests, possibly a vestigial evolutionary advantage.

Some possess a prodigiously acute sense of smell. Helen Keller could smell approaching storms hours before they arrived, sensing a “throb of expectancy” as the earth “exhaled” vapors due to dropping pressure. This explains why perfume smells strongest before storms and at altitude. Keller could decipher house scents and claimed to identify adults’ “personality scent,” describing young men’s scent as elemental, connecting it to “fire, storm, and salt sea” (45).

Ackerman visits the offices of International Flavors & Fragrances (IFF), a company that creates famous perfumes and scents for products from kitty litter to museum exhibits. She interviews Sophia Grojsman, a world-class “nose.” Grojsman compares creating a fragrance to composing music from “accords”—scent combinations that she likens to musical chords. She claims that the part of her brain dedicated to smell “turns off” outside work; she often fails to notice burning food. Many popular perfumes like Opium are variations on classics like Youth Dew. She wants to create a perfume so seductive “no woman could be resisted,” stating that this ambition is not professional but “strictly a female feeling” (54).

Our need for smell is an evolutionary holdover from a time when humans were more vulnerable to natural dangers and thus more attuned to nature. Most of the natural smells humans find pleasant are plants’ chemical defenses or sexual attractants. What is a necessary means of survival for the plant is a source of pleasure for humans, who no longer belong to “the realms of Nature that survive by smell” (55). The quest for spices and perfumes motivated exploration, including Columbus’s voyages, and spread language along trade routes. Perfume originated in Mesopotamia as incense, a burnt offering. The word’s Latin roots—per (through) and fumar (to smoke)—reflect sacred smoke traveling from Earth to the divine.

Ackerman traces the historical development of scent as a marker of power. Egyptians developed enfleurage—using flowers to perfume the body—and the art of the bath. Egyptian Queen Cleopatra received her Roman lover Mark Antony on a ship with perfumed sails. Greek athletes and Roman gladiators anointed themselves for competition. Louis XV’s court was so drenched in fragrance that servants released scented doves at dinner. Christianity and Puritanism brought restraint, but scent always returned. The Koran states that heaven is populated by houris—companions made entirely of sandalwood: pure smell, pure pleasure.