"Is your reality real, or just electrical pulses?"
Proposed by Hilary Putnam, this thought experiment updates Descartes' Evil Demon. Imagine your brain has been removed from your body and placed in a vat of nutrients. A supercomputer connects to your neurons, feeding you electrical signals that perfectly simulate the experience of reading this text, sitting in a chair, and feeling the air.
Could you tell the difference? No. All your sensory experiences occur in the brain. If the inputs are simulated perfectly, the 'reality' feels identical. This inspired the movie The Matrix. It forces us to question: How do we know the external world exists?
This challenges 'Realism.' If we cannot distinguish reality from simulation, can we ever truly 'know' anything about the external world? It suggests our knowledge is limited to our own subjective perceptions.