It’s a glorious spring morning in Sydney and I’m full of nervous anticipation as I cross the university campus, heading toward the lecture theatre where I’m going to be talking to the latest group of students about the senses. I love to watch their faces when I describe the wonders of sensory biology. I want to do it justice; I’m not just relaying information, I’m giving a performance in the hope that my enthusiasm might kindle theirs. On my way, I cut through a Sydney landmark known as the Quadrangle – the centrepiece of the campus. The architects added a finishing touch, a subtropical tree in one corner, and, each year, as the southern hemisphere spring takes hold, this jacaranda tree erupts into bloom, its fragrant lilac flowers calling time on the academic year. Jacarandas across Sydney join in, transforming the city. For a month, the parks and pavements are blanketed with petals. For me, it’s the sensory highlight of the year. I’m aware of a host of other sensations, too. An Australian magpie perched on one of the buildings that surround the Quad. Its oddly metallic call sounds like a steampunk version of the songbirds I grew up with in England. At the same time, I can feel the morning breeze coming in from the Pacific through the archway on the east side of the Quad. My mouth is filled with the warming flavour of one of the aniseed lozenges I rely on for a clear voice in each lecture. The changing stream of sensations provides our perceptual link to the world, a multiplicity of messages that come together to write the autobiography of every second of our lives. For all that our perception seems like a coherent, singular sensory experience, it’s a harmony of many distinct yet compounded senses. The question of just how many senses still lacks a definitive answer – 23 centuries since the first reasoned attempt was made. The Greek philosopher Aristotle is justly regarded as one of the most influential thinkers in history. It’s been said that the science of biology sprang from his labours and many things that he described over 2,000 years ago have stood the test of time. It was Aristotle who came up with the five categories of the senses (or, more formally, sensory modalities): vision, hearing, taste, smell and touch. But that was not the whole story. We certainly have more than five and depending on how we slice and dice the different categories, we might have as many as 53. Touch, for instance, is a composite of multiple different senses that could be subdivided, then there are others such as equilibrioception (the sense of balance) and proprioception (our sense of our body’s position) that lie outside the original five. Generally speaking, a sense can be defined as a faculty that detects a specific stimulus by means of a receptor dedicated to that stimulus. For example, when light enters our eye, it is absorbed by a molecule known as a retinal, which is found within the light-receptor cells of the retina. Taste receptors, meanwhile, coat our tongues, the inside of our cheeks and the very top of the oesophagus. Give them a molecule and, milliseconds later, they’ll be telling the brain all about it. We also have taste receptors sprinkled around the body in places such as the liver, the brain and even the testes. The brain is the seat of all your knowledge, emotions and personality; it’s the home of your innermost thoughts and the place where you experience everything in your life. It has no sensations of its own, yet this is where all our experiences occur. The result of all the work that the brain does in sifting, ordering and processing the incoming information is known as perception. The brain doesn’t simply collect and organise data, it actively regulates and conditions. Signals from the outside world are interpreted and layered with biases, prior expectations and emotions. This integration of sensations and sensibilities plays a powerful role in our perceptions. Many years ago, on the only occasion that they set foot outside Britain, my grandparents travelled to Vienna. It had always been my gran’s dream to visit the beautiful city, to see its architecture, to taste sachertorte, to hear the famous waltzes in their birthplace. Later, she recounted how they’d walked around the corner of a building and come across the famous river that bisects the city. “Look, Jim! The Danube!” she called out in her excitement. “They say that if you’re in love, it looks blue.” My grandad wasn’t a man easily stirred by poetry. His Yorkshire vowels as flat as the cap he habitually wore, he replied tersely, “Looks bloody brown to me.” While common sense might dictate that the Danube could never look blue, there is a nugget of truth to this. When we’re emotionally aroused, activity increases in the brain’s visual cortex, and what we see becomes richer and more brilliant. Ultimately, the convincing perception of reality that we each enjoy is actually a highly complex illusion. Experiences feel real to each of us, yet not one is objectively correct. That doesn’t stop people trying to argue that their subjective perception trumps that of others. This shading of different realities is only the start. It gets more fascinating – and much weirder. It’s one thing to allow that there might be an alternative perspective on colour, but quite another to accept that colour doesn’t actually exist outside our brains. Not only is there no colour, but there’s also no sound or taste or smell. What we perceive as red, for example, is just radiating energy with a wavelength of around 650 nanometres. There’s nothing intrinsically red about it; the redness is in our heads. What we think of as sound is just pressure waves, while taste and smell are no more than different conformations of molecules. Although our sense organs do a splendid job of detecting each of these, it’s the brain that construes them, converting them into a framework for us to understand that world. Valuable though this framework is, it’s an interpretation of reality and, like all interpretations, it’s subjective. With so much information flowing in, demanding immediate attention, how does the brain keep up with it all? The answer is that it doesn’t. It filters the information in its perpetual quest for what’s important. If you’re sitting down now, you’re not likely to have registered the pressure of the chair against your back, or the clothes against your skin – at least until you read this sentence. This isn’t the brain being lazy, it’s separating the important from the irrelevant. The downside is that the brain often misses subtleties, which is how dextrous magicians manage to fool us. The normal workings of the human brain depend on many different sensory inputs, so what happens when these are taken away? Recently, I visited a sensory-deprivation chamber in Sydney’s eastern suburbs. For the most authentic experience, I was told, I’d need to be fully undressed, to avoid the sensation of clothing against my skin, which might put a barrier between me and the bliss that awaited. And so it was that I found myself stark naked and self-consciously stepping into an egg-shaped pod, before pulling the lid closed and embracing sensory oblivion. I lay down, my weight supported by a shallow pool of super-saline water at the same temperature as my blood and with ear plugs to still the faint noises from without. At first, my main emotion was a kind of fretting boredom, my mind chiding me like a fractious child for the withdrawal of stimulus. Once that passed, it switched to standby and I relaxed, but in the absence of anything to see, my mind started to conjure things – flashes of light, geometric patterns that fizzed to life and then shrank to nothingness. This is known as the Ganzfeld effect, or, more evocatively, “the prisoner’s cinema”. It’s been experienced by miners trapped in the dark underground, and by polar explorers whose entire visual field may consist of a uniform white. In ancient Greece, there are records of philosophers descending into caves to induce these hallucinations, in the hope of gaining insight. Given time, the light show can sometimes develop into more fantastical waking dreams. Underlying all of this is the brain’s frantic efforts to build its internal model, even though the sensory information it needs to construct that model has been cut off. The results are odd, though to some they can feel disturbingly real. But what is reality, and, more generally, what does it mean to be alive? However we might try to answer this, it’s fair to say that even our most eloquent attempts fall short of fully conveying the ridiculous, magnificent, miraculous experience of being. Our senses are at the heart of all this wonder. They are the interface between our inner selves and the outside world. They equip us to perceive beauty, from great art to the grandeur of the natural world, and to appreciate a sip of an ice-cold drink and the sound of laughter. Senses are, in short, what make life worth living.