  Quantum decision theory is introduced here, and a new basis for this theory
is proposed. It is first based upon the author's general arguments for the
Hilbert space formalism in quantum theory, and next on arguments for the Born
rule, which is the basis for calculating quantum probabilities. A basic notion
behind the quantum theory foundation is that of theoretical variables, that are
divided into accessible and inaccessible ones. This is here specialized to
decision variables. It is assumed that each accessible variable can be seen as
a function of a specific inaccessible variable. Another assumption is that
there exist two maximal accessible decision processes in the given situation.
Two basic assumptions behind the Born rule are 1) the likelihood principle, 2)
the actor in question has motivations that can be modeled by a hypothetical
perfectly rational higher being. The theory is illustrated by a medical
example. Finally, a broad discussion of decision processes is given.
