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Exactly how many senses do we really have?
While the notion that people have five basic human senses is often considered a universal truth and can be traced back to Aristotle's "De Anima" ("On the Soul"), many philosophers and neuroscientists are now debating whether we may have anywhere from 22 to 33 different senses.
Among these lesser-known senses are equilibrioception, which is associated with our sense of balance; proprioception, which enables us to know which parts of our bodies are where without looking; and chronoception, how we sense the passing of time.
And that's just humans. "There are senses that other species have that we don't, like directional senses and magnetic senses," says Tok Thompson, professor (teaching) of anthropology. Iron oxide in the abdomen of honey bees, for example, can detect changes in the Earth's magnetic field that enable them to navigate to their hive.
The 'X' sense
Then there is the hotly debated existence of the so-called "sixth sense."
"The 'sixth sense' usually refers to an 'unknown' sense, but now that we know there are more than five senses, the idea could perhaps be better thought of as the 'x sense,' where 'x' equals the unknown—whether some yet undiscovered natural sense, or something more along the lines of psychic abilities," says Thompson.
In some Indian philosophies, the mind, or "manas," itself is considered a sixth sense that coordinates the five primary senses with other mental faculties.
Western societies generally equate the sixth sense with extrasensory perception—something that in Celtic culture is traditionally known as "second sight." Among the supposed powers of those with the gift was the ability to predict death, even seeing fish scales (implying a watery grave) appear on someone they sensed would drown soon.
Do a little online digging into the sixth sense and you will find myriad claims of life-changing premonitions that appear to defy the idea that our five senses are our only faculties of perception.
Among them is the inner voice that Wall Street executive Barrett Naylor claimed saved his life—twice. According to a 2009 book about premonitions by physician Larry Dossey, Naylor was heading to work at the World Trade Center on Feb. 26, 1993, when something told him not to go into the building. Later that day, it was bombed. That same instinct, Naylor claimed, also saved him from the 9/11 terrorist attacks in 2001. But a belief in mystical, psychic abilities isn't required to believe that our perceptions can extend beyond the physical senses.
"There are many layers to the mind," says University Professor Antonio Damasio, professor of psychology, philosophy and neurology, and David Dornsife Chair in Neuroscience. "Facts, actions, feelings—all those contents can be retrieved, but are not equally accessible. Some are essentially shrouded in darkness, making conscious access to them difficult."
As a result, Damasio notes, our unconscious mind can suddenly provide us with an answer when we least expect it.
"Intuition is not a myth; it calls attention to the richness of our minds," he says.
Seeing what others don't can sometimes require a catalyst. In many indigenous cultures, psychedelics are ingested to induce ecstatic experiences that bring revelations. The mushroom Psilocybe mexicana is regarded with such awe by Mexico's indigenous communities that the Aztecs dubbed it "teonanacatl," or "God's Flesh."
Visions, primarily those brought on by lysergic acid diethylamide, or LSD, have also been the focus of scientific study. A chemical compound accidentally invented by Swiss researcher Albert Hofmann in 1938, LSD induces powerful, emotional hallucinations.
Of course, LSD eventually found its way into the counterculture, and its ability to inspire visions of an interconnected world helped power the nascent environmental movement of the 1960s. However, the drug's potent effects also triggered numerous tragic outcomes.
Researchers are now evaluating the potential for microdoses of other hallucinogenic substances such as psilocybin mushrooms to help people suffering from PTSD, depression and anxiety.
Seeing is believing
Not all visions require chemical prompting, however. Catholic saints who claimed to have striking visual or auditory hallucinations often lived piously in religious orders. Saint Catherine of Siena is said to have had her first visitation from Christ at the age of 5 or 6. Those in deep, meditative states sometimes tell of encounters with light beings or other strange, visual phenomena.
Nowadays we might assume that these sorts of visions, if not induced by mind-altering substances, are the product of a brain disorder like schizophrenia. But even in medieval times, the church questioned the authenticity of such visions and imposed strict rules to evaluate a vision's legitimacy.
"This is well before psychiatry, of course, but people living during this era still recognized crazy when they saw it," says Lisa Bitel, Dean's Professor of Religion and professor of religion and history. "If someone was mentally ill, their visions would not be taken seriously. You had to be well-regarded in the community and whatever you saw had to be doctrinally correct. A claim that you saw the Virgin Mary riding in on [the medieval equivalent of] a celestial skateboard wouldn't be taken seriously."
For the devout, the modern argument that seeing angels is actually evidence of insanity, or at the very least the result of an overactive imagination, appears absurd. The devout believe that a person who has spiritual visions has gained special access to the truth, not lost their mind. "They would say that the enlightened person is actually seeing things as they really are," says James McHugh, professor of religion.
Visions are still a regularly occurring phenomenon, despite our supposedly more rational approach to the world, says Bitel. Today's reported sightings of UFOs, sometimes said to be inhabited by celestial beings bearing wise words, might just be the descendants of yesteryear's angels.
"Cultural terms may have changed, but the apparitions go on," she says. "The visions we hear about are just the tip of the iceberg."