The authors would like to thank Janne Paanajärvi, Heikki Hyötyniemi, Kai Zenger, Sampsa Laine, Harri Valpola, and Markus Harva for fruitful discussions. This work was supported in part by the Finnish Centre of Excellence Programme (2000-2005) under the project New Information Processing Principles and by the IST Programme of the European Community under the PASCAL Network of Excellence, IST-2002-506778. This publication only reflects the authors' views.