The idea that Jefferson thought of blacks as only 5/7 human is, of course, a canard.  The representatives of the Northern States were trying to have a union without slavery, and they rightly resented the Southern States have slaves be represented the population for purposes of national representation, when they weren't even free and couldn't vote in State elections.  It was like allowing California's illegal alien population to count toward how many representatives it should be allowed in Congress.  So they reached the 5/7 compromise.