On 2 September 2010, Karen Throsby became the 1,153rd person to swim the Channel, taking 16 hours and nine minutes, and keeping herself going on handfuls of jelly babies passed down by her support team. Many Channel swimmers are purists: wetsuits are banned, never mind performance-enhancing drugs. The sport sees itself as an assertion of human ability in natural form. Throsby, a sociologist researching the effects of extreme sports, takes a different view.

She was a speaker at last Saturday's Human Limits, a Wellcome Collection symposium linked to its Superhuman exhibition on physical and mental enhancement. The question it investigated was how much technology can add to a human being before it becomes something else – a cyborg, perhaps, or a superhuman, or a post-human, or a transhuman. What are our limits?

Some speakers discussed the "singularity": the idea that in a few years' time we may converge with our technology to the point that some as yet inconceivable superhuman entity emerges. Others highlighted the fear or even disgust we can feel when new inventions threaten our sense of who we are; uneasy about our authenticity, we look back nostalgically to an era assumed to be simpler, more human.

Throsby's contribution was to remind us that even something as elementally "human" as marathon swimming involves many artificial techniques: gaining weight, acclimatising to the cold, monitoring one's psychology, and developing new micro-senses – an awareness of tiny differences in water temperature, a heightened kinetic sense of the body's balance and position, and so on. It means self-transformation, and is filled with "uncountable, mundane bodily technologies". Channel swimmers use rubber caps, sunblock, Vaseline to prevent chafing, sleek swimsuits and energy-boosting snacks. They are accompanied by boats with GPS.

And they use goggles, an invention variously attributed to Polynesians, Persians and the Inuit, but later improved by innovators such as first female Channel swimmer Gertrude Ederle, who smeared paraffin wax on motorcycle glasses in 1926 to make them watertight. More recently, goggles have been made with better rubber, adjustable straps and prescription lenses. It would be hard to swim far or fast without them.

As always, successful technologies tend to disappear in their use, becoming almost indistinguishable from ourselves and our own efforts. A smartphone sits in our hand announcing: "I am technology", but the spectacles through which we peer at its screen, the pocket into which we slip it and the heel with which we stamp it into the ground in a rage if it malfunctions all feel as natural as our own hands and eyes. It takes a leap of thought to realise that Vaseline and jelly babies are technology, too.

Human Limits asked how much technology we can add before losing ourselves, but there is also the question of how human we remain if familiar enhancements are taken away: not just devices but practices – our mastery of writing, our elaborate educations, our knives and fires and cooking-pots, our language, our laboriously polished social skills. At what point do we cross the line into being no longer ourselves?

As human beings, we tread a narrow ridge where we roughly know who we are, with landscapes of mystery, anxiety and ambiguity on each side. But the ridge does not run straight, or lead in a predefined direction. It is partly up to us to decide what a human being is.

"Man is rightly called and judged a great miracle and a wonderful creature," wrote philosopher Pico della Mirandola in 1486, opining that we are wonderful not because we live up with the angels, or down with more modest beasts, but because we occupy an intermediate realm in which we invent and alter ourselves. "Neither a fixed abode nor a form that is thine alone nor any function peculiar to thyself have we given thee," he imagines God saying to man. "Thou, constrained by no limits, shalt ordain for thyself the limits of thy nature."

Of course we are hemmed in by mishaps and errors, and technology goes wrong. But to a large extent we are our own works in progress. And when all goes smoothly, we don't even know it.