  We introduce a novel real-valued endogenous logic for expressing properties
of probabilistic transition systems called Riesz modal logic. The design of the
syntax and semantics of this logic is directly inspired by the theory of Riesz
spaces, a mature field of mathematics at the intersection of universal algebra
and functional analysis. By using powerful results from this theory, we develop
the duality theory of the Riesz modal logic in the form of an
algebra-to-coalgebra correspondence. This has a number of consequences
including: a sound and complete axiomatization, the proof that the logic
characterizes probabilistic bisimulation and other convenient results such as
completion theorems. This work is intended to be the basis for subsequent
research on extensions of Riesz modal logic with fixed-point operators.
