I found the discussion in the thread appended to Is There a 'No God' Delusion? very stimulating and useful. My man Peter is the 'rock' upon which good discussions are built. (I shall expatiate later on the sense in which Lupu is also a 'wolf.') The thread got me thinking about what exactly a delusion is. It is important that I have an explicit theory of this inasmuch as I routinely tag leftist beliefs as delusional.
If belief is our genus, the task is to demarcate the delusional from the illusory species and both species from beliefs in general. In this context, and as a matter of terminology, a delusion is a delusional belief, and an illusion is an illusory belief. (I won't consider the questions whether there are illusions or delusions that do not belong to the genus belief.) Let us push forward by way of commentary on some claims in Sigmund Freud's The Future of an Illusion (tr. Strachey, Norton, 1961).
1. Freud distinguishes between illusions and errors. (p.30) Eine Illusion ist nicht dasselbe wie ein Irrtum . . . . There are errors that are not illusions and there are illusions that are not errors. Given that our genus is belief, an error is an erroneous or mistaken belief. So now we have three species of belief to contend with: the erroneous, the illusory, and the delusional. "Aristotle's belief that vermin are developed out of dung . . . was an error." (30) But "it was an illusion of Columbus's that he had discovered a new sea-route to the Indies." (30) What's the difference? The difference is that illusions are wish-driven while errors are not. "What is characteristic of illusions is that they are derived from human wishes." (31) Für die Illusion bleibt charakteristisch die Ableitung aus menschlichen Wünschen . . .
2. Every erroneous belief is false, but no erroneous belief is derived from human wishes. Every illusory belief is derived from human wishes, and may be either true or false. So if a belief is illusory one cannot infer that it is false. It may be false or it may be true. By 'false' Freud means "in contradiction to reality." (31) Suppose that a middle-class girl cherishes the belief that a prince will come and marry her. And suppose the unlikely occurs: a prince does come and marry her. The belief is an illusion despite the fact that it is true, i.e., in agreement with reality. The belief is illusory because its formation and maintenance have their origin in her intense wish. The example is Freud's.
3. The difference between an illusory belief and a delusional belief is that, while both are wish-driven, every delusional belief is false whereas some illusory beliefs are true and others false. "In the case of delusions we emphasize as essential their being in contradiction with reality." (31) An der Wahnidee heben wir als wesentlich den Widerspruch gegen die Wirklichkeit hervor, die Illusion muß nicht notwendig falsch, d. h. unrealisierbar oder im Widerspruch mit der Realität sein. To sum up:
Errors: All of them false, none of them wish-driven.
Delusions: All of them false, all of them wish-driven.
Illusions: Some of them false, some of them true, all of them wish-driven.
4. Now that we understand what an illusion is, we are in a position to understand Freud's central claim about religious ideas and doctrines: "they are illusions, fulfillments of the oldest, strongest, and most urgent wishes of mankind." (30) ". . . all of them are illusions and insusceptible of proof." (31) Sie sind sämtlich Illusionen, unbeweisbar, . . ..
To say of a belief that it is an illusion is to say something about its psychological genesis or origin: it arises as the fulfillment of a wish. It is not to say anything about the belief's truth-value (Wahrheitswert). So even if some religious doctrines were susceptible of proof, they would still be illusions. For again, what makes a belief an illusion is its stemming from a wish. Since Freud admits that there are true illusions, he must also admit at least the possibility of there being some provably true illusions. It could therefore turn out that the belief that God exists is both demonstrably true and an illusion.
But although this follows from what Freud says, he does not explicitly say it. Indeed, he says something that seems inconsistent with it. After telling us that "the truth-value of religious doctrines does not lie within the scope of the present inquiry," he goes on to say that "It is enough for us that we have recognized them as being, in their psychological nature, illusions. But we do not have to conceal the fact that this discovery also strongly influences our attitude to the question which must appear to many to be the most important of all." (33) That question, of course, is the question of truth or falsity.
So the good Doktor appears to be waffling and perhaps teetering on the brink of the genetic fallacy. On the one hand he tells us that a belief's being an illusion does not entail that it is false. He himself gives an example of a true illusion. On the other hand, from what I have just quoted him as saying it follows that showing that a belief arose in a certain way, in satisfaction of certain psychological needs or wishes, can be used to cast doubt on its truth. But the latter is the genetic fallacy. If a third-grader comes to believe the truths of the multiplication table solely on the strength of her teacher's say-so, this fact has no tendency to show that the beliefs formed in this way are false.