[[http://www.filosofia.org/aut/svc/1922p047.htm|Spencer, Herbert]] - {{:figari:anexos:spencer_h._-_the_principles_of_psychology.pdf|The Principles of Psychology}}. Longman, Brown, Green and Longmans, Londres, 1855 (E-Book in PDF format published by Liberty Fund, Inc., a private, non-profit, educational foundation. From  [[http://oll.libertyfund.org|The Online Library of Liberty]] web site)