Franklin's supposed deism was actually philosophical necessitarianism, which led him to relativism. He saw reason as antagonistic to all religious sentiment (a “Want of Religion”), not only to revealed religion. Desiring to hide this atheism from his readers, he presented himself more palatably as a deist, with the suggestive adjective “thorough.” Lemay writes, “Deism was a comparatively acceptable belief until the end of the eighteenth century, whereas the satire and materialism embodied in the Dissertation were shocking.” Franklin, however, in one of the most intriguing remarks in the Autobiography, claims that he abandoned necessitarianism: “I began to suspect that this Doctrine tho’ it might be true, was not very useful.” With this ambiguous statement, he relates his change of mind, in which he rejected Mandeville's teaching. This change is marked first by an inquiry into the social nature of man, recorded in his journal on his return voyage from London, and second by a serious examination of the chief ends of human language, which revealed what Franklin called the insinuating error of the Dissertation. The Dissertation and its necessitarian argument, he concluded, was itself an a priori argument that had infected his reasoning. He thus rejected its deterministic conclusions as the requisite foundation for his moral philosophy.

“Journal of a Voyage” (1726)

In London, Franklin had squandered his money. Rather than continue there or travel throughout Europe, in July 1726 he accepted an offer by Quaker merchant Thomas Denham to pay his fare back to Philadelphia. The notes of his “Journal of a Voyage” show a change from the thesis of the Dissertation. In these pages, Franklin clearly viewed his prior behavior as problematic. He decided that virtue was necessary after all, both in human relations and for individual happiness. Considering the first, he concluded that none could embrace a vicious way of life and maintain a good reputation. Considering the second, he questioned his proud opinion that he was self-sufficient.

Franklin's trip across the Atlantic was painfully slow; the ship did not reach Philadelphia until October 9—some three months. In preparation for the voyage, it stopped at several ports, where he and the crew observed castles and graveyards.