  Probability theory is far from being the most general mathematical theory of
uncertainty. A number of arguments point at its inability to describe
second-order ('Knightian') uncertainty. In response, a wide array of theories
of uncertainty have been proposed, many of them generalisations of classical
probability. As we show here, such frameworks can be organised into clusters
sharing a common rationale, exhibit complex links, and are characterised by
different levels of generality. Our goal is a critical appraisal of the current
landscape in uncertainty theory.
