In his minute, detailed descriptions of life in colonial America, de Crèvecoeur makes general observations about humanity as a whole. Initially, these observations are generally positive, as the fictitious farmer James expresses to his English correspondent a sense of profound joy, pride, and wonder at life in the American colonies.
Believing that people are largely determined by their physical surroundings, James attributes the American character, with its fortitude, individualism, and hardy self-reliance, to the natural environment, which creates a very special kind of person.
As the letters progress, however, James starts to display signs of disillusion with the country in which he lives. Among other things, this means that his belief in environmental determinism takes quite a hit, as he comes to emphasize people's active role in creating themselves and their society.
The catalyst for de Crèvecoeur's change of heart, as expressed through the words of James the farmer, appears to have been his encounter with slavery. Using James as a mouthpiece, de Crèvecoeur doesn't hold back in his withering attack upon the institution of slavery, something he regards as brutal, dehumanizing, and evil.
Somewhat inevitably, his encounter with slavery gives him a more negative attitude towards humanity, one tempered by a good deal more realism than his previous stance.