  These lecture notes provide a basic introduction to the framework of
generalized probabilistic theories (GPTs) and a sketch of a reconstruction of
quantum theory (QT) from simple operational principles. To build some intuition
for how physics could be even more general than quantum, I present two
conceivable phenomena beyond QT: superstrong nonlocality and higher-order
interference. Then I introduce the framework of GPTs, generalizing both quantum
and classical probability theory. Finally, I summarize a reconstruction of QT
from the principles of Tomographic Locality, Continuous Reversibility, and the
Subspace Axiom. In particular, I show why a quantum bit is described by a Bloch
ball, why it is three-dimensional, and how one obtains the complex numbers and
operators of the usual representation of QT.
