>Dragan suggested that we define an observer class along side the particle class.

I only assumed there should be an observer class. Particle class is yours.

>That would mean that the observer is something special, it is not material because it is not made of the particles of matter.

Now that's another assumption, that we should have particle objects... and then have to decide whether our observer is material. The models would bifurcate from here. Makes it more interesting.

>Your suggestion was to put an Observe() method right on the particle class, making the particle the observer. That makes the observer material, but it doesn't really explain how a human observer fits into the model. The human being is obviously not one particle, but it is an observer.
>
>So if the particle has the observe method, how does a human being like myself observe?

procedure observe()
oObservation=create("empty")
for each oParticle in this.collParticles
   oParticle.observe()
  addProperty(oObservation, oParticle.cPropName, oParticle.uPropValue)
endfor
return oObservation

In the universe where observer is also a particle, particle would be a collection. In the other one, particle can be anything, but observer is a collection (or something more complex), or has a collection of its own particles.