The wording actually assumes they are full persons. It distinguishes between the contribution to the census from free persons and the contribution from other persons. It's 3/5 of the number of other persons that gets added to the number of free persons. It's not that slaves are 3/5 of a person.
And for the record, it was those who opposed slavery who didn't want them counted and those who favored it who did, because counting them as full persons would mean more representation in Congress for their states (and yet the voting for those states wouldn't involve the slaves voting, of course, so it's even more influence for the slave-holders if they counted fully).
Friday, April 02, 2010
Jeremy has a good post addressing misconceptions about the 3/5 clause in the original Constitution: