G. Engels, R. Heckel, and J.M. Küster. 4th International Conference on The Unified Modeling Language UML 2001, volume 2185 of Lecture Notes in Computer Science, page 272--286. Springer-Verlag, Berlin, (2001)
Karl Marx, and Friedrich Engels. Progress, Moscow, (1976)The premises from which we begin are not arbitrary ones, not dogmas,
but real premises from which abstraction can only made in the imagination.
They are the real individuals, / their activity and the material
conditions of their life, both those which they find already existing
and those produced by their activity. The premises can thus be verified
in a purely empirical way. Marx \& Engels 1976, 36-37.