Undecidable classical properties of observers
  A property of a system is called actual, if the observation of the test that pertains to that property, yields an affirmation with certainty. We formalize the act of observation by assuming that the outcome correlates with the state of the observed system and is codified as an actual property of the state of the observer at the end of the measurement interaction. For an actual property, the observed outcome has to affirm that property with certainty, hence in this case the correlation needs to be perfect. A property is called classical if either the property or its negation is actual. It is shown by a diagonal argument that there exist classical properties of an observer that he cannot observe perfectly. Because states are identified with the collection of properties that are actual for that state, it follows that no observer can perfectly observe his own state. Implications for the quantum measurement problem are briefly discussed. 
Sven Aerts