  In computer science, especially when dealing with quantum computing or other
non-standard models of computation, basic notions in probability theory like "a
predicate" vary wildly. There seems to be one constant: the only useful example
of an algebra of probabilities is the real unit interval. In this paper we try
to explain this phenomenon. We will show that the structure of the real unit
interval naturally arises from a few reasonable assumptions. We do this by
studying effect monoids, an abstraction of the algebraic structure of the real
unit interval: it has an addition $x+y$ which is only defined when $x+y\leq 1$
and an involution $x\mapsto 1-x$ which make it an effect algebra, in
combination with an associative (possibly non-commutative) multiplication.
Examples include the unit intervals of ordered rings and Boolean algebras.
  We present a structure theory for effect monoids that are $\omega$-complete,
i.e. where every increasing sequence has a supremum. We show that any
$\omega$-complete effect monoid embeds into the direct sum of a Boolean algebra
and the unit interval of a commutative unital C$^*$-algebra. This gives us from
first principles a dichotomy between sharp logic, represented by the Boolean
algebra part of the effect monoid, and probabilistic logic, represented by the
commutative C$^*$-algebra. Some consequences of this characterisation are that
the multiplication must always be commutative, and that the unique
$\omega$-complete effect monoid without zero divisors and more than 2 elements
must be the real unit interval. Our results give an algebraic characterisation
and motivation for why any physical or logical theory would represent
probabilities by real numbers.
