In her Discourse on Happiness, Émilie du Châtelet argues susceptibility to illusion is one of the five ‘great machines of happiness,’ and that ‘we owe most of our pleasures to illusions’. However, many who read the Discourse find this aspect of her view puzzling and in tension with her claims that we must always seek truth and obey reason. To understand better her claims in the Discourse on Happiness, this article explores Du Châtelet's discussions of illusions in her Foundations of (...) Physics, On Liberty, and the Dissertation on the Nature and Propagation of Fire. I distinguish four types of illusions that Du Châtelet posits and clarify the ways in which these relate to her views on happiness and love in the Discourse and argue that she avoids deceptive or perpetual illusions of happiness through the use of the principle of sufficient reason. (shrink)