Philosophers have long posed thought experiments to examine the nature the interaction between our internal 'self' and external 'reality', questioning how much we can trust the evidence of our senses.
The idea runs that we are living in some kind of illusory universe - whether that be the product of an 'evil demon' as in Descartes, or a complex computer simulation like The Matrix.
While still largely the preserve of science fiction, the idea that the world we experience as 'real' is actually just a simulation is still given serious thought by scientists, philosophers, and even businesses.
Developments in our understanding of quantum mechanics have reinforced this idea, with the boundary between the 'world out there' and our own subjective consciousness becoming blurred.
One report, issued by the Bank of America in September 2016, gave a 20-50 per cent chance that we ourselves are living in some kind of Matrix-like simulation.
Philosopher Bill Nye is pondering whether we could be living in a simulated world (below).