In order to be happy, one must have freed oneself of prejudices, one must be virtuous, healthy, have tastes and passions, and be susceptible to illusions; for we owe most of our pleasures to illusions, and unhappy is the one who has lost them. Far then, from seeking to make them disappear by the torch of reason, let us try to thicken the varnish that illusion lays on the majority of objects.--Emilie Du Châtelet, Discourse on Happiness [Discours sur le bonheur.] translated by I. Bour & J.P. Zinsser, p. 349.
At the heart of the so-called 'age of reason,' we find an embrace of the utility of illusions. For, according to Du Châtelet illusions are necessary to feel certain pleasures, and these pleasures ("satisfied tastes and passions") make us happy. In order, then, to enjoy some tastes and passions one must be in the grip of certain illusions. It's clear from what follows that according to Du Châtelet not all tastes and passion require illusions. Some goods like -- food or certain 'sensual pleasures' -- can be enjoyed without illusions (but see **). So, on her behalf, I'll distinguish between simple pleasures and complex pleasures. It's only the consummation of complex pleasures that require illusions in order to be enjoyed.
Before I get to the nature of complex pleasures, it is worth marking Du Châtelet's distinction between prejudices [préjugés], which are to be emendated, and illusions. A prejudice "is an opinion that one has accepted without examination, because it would be indefensible otherwise." (p. 352) All inherited tradition, which is tacitly or explicitly assumed correct, falls under this definition; it is no surprise that religion is Du Châtelet's exemplar of a prejudice. While Du Châtelet is clearly some kind of (Leibnizian) Deist, she is no friend of revealed religion which she thinks makes people unhappy (in this life). As we know from her commentary on Scriptures, she finds these texts to be full of contradictions and absurdities. But the key point for Du Châtelet is that it only requires ordinary intelligence and modest education to correct prejudices. Prejudices are not useful (to healthy souls).*
According to Du Châtelet, an illusion is "not an error." For errors are "always harmful" (354). She defends the "love of study" as a sure means to happiness, especially in independent and magnanimous souls (357--some other time I discuss her self-attribution of magnanimity and her analysis of glory in this context). So, an illusion is a defensible opinion, which gives us an "agreeable feeling" accompanying complex tastes and passions.** Such an opinion one has accepted after scrutiny, even though it's possible, even likely, to be false, but one chooses not to destroy their functionality (355). On an epistemic level Du Châtelet's category of an illusion is close in spirit to the character of Spinoza's universal tenets of faith, which can be beyond rationally knowable, but ought to avoid knowable falsehood. (Of course, Du Châtelet's illusions have a different social role than Spinoza's tenets.) While one can choose to correct or maintain an illusion, one cannot control becoming in the grip of one; it's involuntary and relies on natural mechanisms (physiology, imagination, etc.); in this context she speaks of the "great machines of happiness." She offers as an exemplar optical illusions, which tend not to disappear even when one knows quite a bit about optical science.
Love is sustained by such necessary illusions according to Du Châtelet. Now, it seems that when speaking of 'love,' Du Châtelet has in mind loving another and not being loved, although the fact that one's love may be reciprocated is necessary part of love. That is, she is writing from first-personal perspective and not analyzing the relation or relationship of love. Love involves a kind of pleasure that follows from a giving oneself over to one's feelings of tenderness. Being tender to another feels pleasing, and this just is love. Such tenderness comes in degrees: the love she feels for a plant (which is barely capable of reciprocation) is different from the love she felt for Voltaire. In fact, it's quite clear from context is that Du Châtelet is more interested in passionate tenderness than a wholly calm tenderness.
Love has two further characteristics: (i) it makes "us wish to live"++ and (ii) it promotes gratitude not to the one that loves us, but to God or Nature.+ That is to say, love indirectly makes us aware of and receptive to something larger than our human selves. For lack of a better word, I'll use 'cosmos' here.
- So, love is a pleasing, passionate tenderness for another that makes us wish to live and makes us receptive to the cosmos.
Interestingly enough, Du Châtelet notes that there can be excessive love such that there is no space for reciprocation. And she thinks men in her social class get turned off by excessive love.
But what, then, are the illusions that accompany love? On this point Du Châtelet is rather terse, despite the fact that she has quite a bit to say about love and, especially, how to comport oneself after a beloved loses interest. This is, I think, also a clue: in love, one must believe that it will last forever.
*She allows that corrupted souls may find prejudices very useful.
**At one point Du Châtelet claims that illusions are involved in all pleasures of life. But I do not think she needs this strong claim.
+ I use the Spinozistic expression. Du Châtelet uses "the author of nature, whoever he is."
++In the piece she quite clearly thinks that suicide is legitimate.