In the eighteenth century, many Europeans and Americans embraced a world-view in which the natural world was seen as complete, full, and perfect, as created by God. Within this worldview, no species ever became extinct because such an event would destroy the perfection of nature. Toward the end of the eighteenth century, the concept that no species had ever become extinct was increasingly challenged by evidence from the fossil record. By the early nineteenth century, a new paradigm, the “former-worlds” view of Earth history, began to emerge. Buffon had argued that New World quadrupeds were degenerate varieties of Old World species, and that at least one of them had gone extinct. The idea of New World degeneracy thus became connected with the concept of extinction. Thomas Jefferson conducted a long, personal campaign to discredit these ideas of Buffon’s, arguing against them in the early 1780s in Notes on the State of Virginia and also in his 1797 Megalonyx memoir. Jefferson resisted the concept of extinction for a very long time, and he was never able to let go of his “completeness-of-nature” worldview. I suggest that several factors contributed to Jefferson’s inability to relinquish his worldview, in spite of the fact that there was considerable empirical evidence showing that it was not valid. The most influential factors were (1) Jefferson’s emotional and public commitment to the completeness-of-nature worldview, and (2) Jefferson’s personality traits, which were acquired in part through his experiences as an eldest son.