It has become one of the defining questions of our age. Are you a "laurel" or a "yanny"? Not sure? Click here to find out (even if you have heard it before, things may have changed...).

This week, the recording at the start of that clip went viral, sparking a fierce debate which, beneath the surface, explores fundamental questions about how we experience the universe.

It all starts with "laurel" and "yanny". Which one did you hear? Chances are you have friends who heard the opposite. But how can one sound can be interpreted in two different ways?

Science has the answer. The recording is of the word "laurel", but it contains both high and low frequencies. It is these higher frequencies (and the patchy sound quality) which make some people hear "yanny" - it depends on which frequency your brain focusesEverybody's brain activity is slightly different. In this case it could depend on age, a listener's past experiences or what they are expecting to hear. The type of speakers or headphones used will also make a difference. on.

But instead of settling the issue, this explanation only leads to harder questions.

If our senses can deceive us, how can we rely on them? For that matter, can we be sure that what we experience is real? And does this mean that everyone has their own different version of "reality" whirring inside their brains? But this would mean that... there is no such thing as "reality" at all?

These questions have vexed philosophers for millennia.

Philosophical scepticism is the school of thought that true knowledge is impossible to attain. Among the first sceptics was an ancient Greek philosopher called Pyrrho. Born in 360 BC, he believedHe produced no known written records, however his thoughts were recorded and passed down by followers and other writers. that our senses reveal nothing about the true nature of the world.

Think about colour. To one person, a basket of mixed apples will appear red and green. To someone with colour blindnessThe condition can manifest in several different ways, however red-green colour blindness is the most common form., the same apples would all look a brownish-green. To a hungry bat they will be shades of black and white. Colour exists in the mind of the observer - not in the apples themselves.

Other philosophers like Rene Descartes have argued that "reality" could actually be all a vivid dreamKnown as the dream argument, the hypothesis was also explored by Plato and Aristotle., stating we cannot rely on senses to disprove this claim. This idea had a modern re-imagining in the film The MatrixRather than a dream, the film explores a scenario in which the world as humanity knows it is actually a massive computer simulation maintained by an advanced robot civilisation..

Most probably, some argue. There is no proof otherwise, and in philosophy it is usually the simplest explanation that is correct. Furthermore, each person can be pretty sure they exist, as Descartes himself said: "I think, therefore I am." Plus there must be a "real" physical world for our senses to interact with. Case closed.

Do not be so sure, others respond. This quirky recording reminds us that all experience is subjective. And if that is the case, then no solid sense of reality can be said to exist. What's more, every possible sensation is necessarily filtered and staged by our imperfect minds. All is illusion.