  In a Generalised Probabilistic Theory (GPT) equipped additionally with some
extra geometric structure we define the morphophoric measurements as those for
which the measurement map transforming states into distributions of the
measurement results is a similarity. In the quantum case, morphophoric
measurements generalise the notion of a 2-design POVM, thus in particular that
of a SIC-POVM. We show that the theory built on this class of measurements
retains the chief features of the QBism approach to the basis of quantum
mechanics. In particular, we demonstrate how to extend the primal equation
(`Urgleichung') of QBism, designed for SIC-POVMs, to the morphophoric case of
GPTs. In the latter setting, the equation takes a different, albeit more
symmetric, form, but all the quantities that appear in it can be interpreted in
probabilistic and operational terms, as in the original `Urgleichung'.
