"The principle of which we were ashamed" refers to slavery. In the writing of the US Constitution, the word slavery was omitted from Article 1, Section 2 which outlined how the states would be represented in Congress according to their population. Each free man counted as one person, and every five slaves counted as three. However, slaves were referred to in the document as "other persons" prompting a moral objection by John Dickinson who wrote the little-known phrase in his personal notebook during the 3/5ths debate at the Constitutional Convention in 1787.