A world object is the instance of the class PasteUpMorph to which the answer of #isWorldMorph is true. To play the role of a world object the PasteUpMorph object keeps a reference to a WorldState object.
The PasteUpMorph object playing the role of a world is accessible for example through
The global object World keeps a reference to it as well. However, this global reference is maintained only for backward compatibility, and message sends should be used for accessing the world (usually either Project>>world or Morph>>world).