"First we will address the capitation (counting) of slaves in the Constitution. On June 11, Roger Sherman suggested that representation be based on a count of all free men.
The South wanted their slaves counted as whole persons, but that would never happen. James Wilson wanted to get the issue out of the way quickly, and asked the Convention to adopt the same standard as that in the Articles: slaves would count as three-fifths persons. "
http://www.usconstitution.net/consttop_ccon.html