Another result of the apparent statistical nature of quantum mechanics is that, when a measurement is made of a system in a mixed state, it must be found to be in one of its allowed eigenstates. This raises the question of whether the observation forces the system to assume an eigenstate, or only reveals the value of a pre-existing state. If the former interpretation is true, then the act of observation is responsible for creating reality. The most famous paradox resulting from this interpretation was proposed by Schrödinger, and has come to be known as the "Schrödinger's cat paradox." In formulating his paradox, Schrödinger considered a cat locked a closed box together with a vial of cyanide. If the vial containing the cyanide has a 50% chance of breaking and releasing the gas in a given time interval, Schrödinger then asked, what is the status of the cat after the elapsed time when an observation is made? Quantum mechanics dictates that, before an observation is made, the cat will be in a mixed state of "half-live" and "half-dead." It is only upon making an observation that the cat's wavefunction must collapse to one or the other eigenstate. It therefore appears that the mere act of observation is responsible for either killing the cat, or yanking him firmly back into the land of the living.

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