Abstract
Usual derivations of Lilders's projection rule show that Liuders's rule is the
rule required by quantum statistics to calculate the final state after an ideal (minimally
disturbing) measurement. These derivations are at best inconclusive,
however, when it comes to interpreting Liuders's rule as a description of individual
state transformations. In this paper, I show a natural way of deriving
Liiders's rule from well-motivated and explicit physical assumptions referring
to individual systems. This requires, however, the introduction of a concept of
individual state which is not standard.