# 1.1: Understanding Nature

- Page ID
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Quantum Theory encompasses our best understanding of how nature works: what will be the result of any experiment. We arbitrarily split the universe into “system” (wavefunction), an environment or measurement (Hamiltonian, or operator) and a measureable quantity (eigenvalue). There is no unique associated mathematics, but since all measurements on systems yield real numbers, we need mathematics which gives real eigenvalues. So it is a premise of quantum theory that any measurable quantity is associated with a Hermitian operator.