Walter's "Cognitive extension: the parity argument, functionalism, and the mark of the cognitive"

Available online here: http://www.springerlink.com/content/h21565731471r11h/fulltext.pdf;.

Sven has a nice discussion of the use of the Parity Principle.

Check out, as well, the other papers in the issue by my buds, Carl, Larry, Jacqueline, John, Philippe, and Tom.  It was edited by Jacqueline Sullivan.