This article investigates the question why even the existence of “moral conscience” became regarded with serious doubts among radical eighteenth-century French philosophes La Mettrie, d’Holbach, Diderot, and Voltaire, from the vantage point of conceptual history. The philosophes’ stance of regarding moral conscience only as a name for certain acquired prejudices both fails to engage with the conception of moral conscience upheld by their theistic opponents and stands in a sharp contrast to the moral thought of Protestant reformation, which – less (...) than two centuries earlier – had elevated moral conscience to a position of an infallible moral authority. The article argues, however, that, in spite of the opposition between these two conceptions of conscience, they might be connected, and that the former might have even been a consequence of the latter. It will be shown that the pre-Reformation conception of moral conscience, crystallized in Aquinas, presents it being based on our natural attraction to “the human good”. However, as this conception was not created by medieval theologians, but was inherited from non-theistic ancient philosophy, in principle it would have been as compatible with the deistic and atheistic worldviews of the philosophes as the other ancient philosophical concepts, such as “the human good” and “human nature”, that the philosophes continued using in their original, teleological senses. Having presented a brief overview of the history of moral conscience, the article proceeds to show how the Protestant reformers removed the attraction to the human good from moral conscience while making its normative guidance infallible. The article will suggest that, due to the prevalence of this, exclusively theological, conception of moral conscience in the intellectual environment shaped by Protestantism, several British thinkers of the seventeenth century, including Hobbes, Locke, and Hume, who could not endorse the theological aspects of moral conscience, gradually reduced it into a mere acquired “sentiment”. As their philosophy was a major influence on the philosophes, the article concludes that the loss of a rational justification for moral conscience in the works of the British philosophers could have also been what motivated the philosophes to question moral conscience. (shrink)