One passage in particular struck me. Stendhal is describing two characters who are falling in love (mostly); both are young, have lived all their lives in a backwater provincial town, and neither has been well educated.
The idea that reading novels could change the way people fall in love might strange today, but remember that in 1830 the novel as we know it was still a fairly new invention, and was seen in conservative quarters as potentially dangerous. Stendhal was of course pro-novels (he was a novelist), but he accepts that they have a profound effect on the minds of readers.
In Paris, the nature of [her] attitude towards [him] would have very quickly become plain - but in Paris, love is an offspring of the novels. In three or four such novels, or even in a couplet or two of the kind of song they sing at the Gymnase, the young tutor and his shy mistress would have found a clear explanation of their relations with each other. Novels would have traced out a part for them to play, given them a model to imitate.
Notice that his claim is not that novels create entirely new emotions. The two characters had feelings for each other despite never having read any. Novels suggest roles to play and models to follow: in other words, they provide interpretations as to what emotions mean and expectations as to what behaviours they lead to. You feel that, therefore you'll do this.
This bears on many things that I've written about recently. Take the active placebo phenomenon. This refers to cases in which a drug creates certain feelings, and the user interprets these feelings as meaning that "the drug is working", so they expect to improve, which leads them to feel better and behave as if they are getting better.
As I said at the time, active placebos are most often discussed in terms of drug side effects creating the expectation of improvement, but the same thing also happens with real drug effects. Valium (diazepam) produces a sensation of relaxation and reduces anxiety as a direct pharmacological effect but if someone takes it expecting to feel better, this will also drive improvement via expectation: the Valium is working, I can cope with this.
The same process can be harmful, though, and this may be even more common. The cognitive-behavioural theory of recurrent panic attacks is that they're caused by vicious cycles of feelings and expectations. Suppose someone feels a bit anxious, or notices their heart is racing a little. They could interpret that in various ways. They might write it off and ignore it, but they might conclude that they're about to have a panic attack.
If so, that's understandably going to make them more anxious, because panic is horrible. Anxiety causes adrenaline released, the heart beats ever faster etc., and this causes yet more anxiety until a full-blown panic attack occurs. The more often this happens, the more they come to fear even minor symptoms of physical arousal because they expect to suffer panic. Cognitive behavioural therapy for panic generally consists of breaking the cycle by changing interpretations, and by gradual exposure to physical symptoms and "panic-inducing" situations until they no longer cause the expectation of panic.
This also harks back to Ethan Watters' book Crazy Like Us which I praised a few months back. Watters argued that much mental illness is shaped by culture in the following way: culture tells us what to expect and how people behave when they feel distressed in certain ways, and thus channels distress into recognizable "syndromes" - a part to play, a model to imitate, though probably quite unconsciously. The most common syndromes in Western culture can be found in the DSM-IV, but this doesn't mean that they exist in the rest of the world.
Like Stendhal's, this theory does not attempt to explain everything - it assumes that there are fundamental feelings of distress - and I do not think that it explains the core symptoms of severe mental illness such as bipolar disorder and schizophrenia. But people with bipolar and schizophrenia have interpretations and expectations just like everyone else, and these may be very important in determining long-term prognosis. If you expect to be ill forever and never have a normal life, you probably won't.