It is claimed that happiness is the motive and the aim of every human act at the same time. Around this issue, so many opinions and ideologies are forged which ponder upon the possible means that allow Man to fulfill his/ her happiness. In the XVIIIth century, the quest of happiness is often laid in an ineluctable philosophic paradox. That is to say, Man wants to be happy, but he /she ignores the means to achieve it. To Voltaire, this paradox starts by the demonstration of the tension between good and bad, and between melancholy and happiness. In his poem « On the natural law », his ultimate objective is to get rid of such tension. For him, Man should dodge every cruel act to seize the opportunities of happiness; this happiness cannot be possible only through a universal morality which unites all human beings such as the execution of the law of natural religion which permits to get rid off of irrational dogma and every insignificant illusion. In this case, Man, the first responsible of his/ her proper happiness, should get away from prejudices and metaphysical ideas in order to have access to the approval of the universal morality translated by the justice and tolerance and not by fanatism and intolerance. The voltairien humanism is in this case an important condition.