[edit: Just for reference - This is actually my own extraction from a larger extraction which appears in Christopher Hitchens' book The Portable Atheist, which is a collection of writings on religion, science, morality and belief from atheists, agnostics and even some theists from the last 2000 years. As the book is arranged chronologically, this essay appears earlier in the book than more recent writings, obviously, thus this is was one of the first essays I've summarized for my own reference. The essays in the rest of the book make excellent points also, and are more recent, however I haven't summarized them for myself yet. Once I do I will try to blog some of the more relevant points of those also. In the meantime, I posted this selection as I thought it made very valid points, regardless of when it was written.]
We turn our attention to the psychical origin of religious ideas. These which are given out as teachings are not precipitates of experience or end-results of thinking: they are illusions, fulfillments of the oldest, strongest, and most urgent wishes of mankind. The secret of their strength lie in the strength of these wishes. As we already know, the terrifying impression of helplessness in childhood aroused the need for protection - for protection through love - which was provided by the father [Keep in mind this was published in 1927.]; and the recognition that this helplessness lasts throughout life made it necessary to cling to the existence of a father, but this time a more powerful one. Thus the benevolent rule of a Divine Providence allays our fear of the dangers of life; the establishment of a moral world-order ensures the fulfillment of the demands of justice, which have so often remained unfulfilled in human civilization; and the prolongation of earthly existence in a future life provides the local and temporal framework in which these wish-fulfillments shall take place. Answers to the riddles that tempt the curiosity of man, such as how the universe began or what the relation is between body and mind, are developed in conformity with the underlying assumptions of this system.
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[What is an illusion?]
An illusion is not the same thing as an error, nor is it necessarily an error. [As an example he states that Aristotle's belief that vermin are developed out of dung was an error, whereas Columbus's belief that he had discovered a new sea-route to the Indies is an illusion.] What is characteristic of illusions is that they are derived from human wishes. [He also makes the distinction that these are similar to psychiatric delusions, but differ in that delusions are necessarily in contradiction with reality.] Illusions need not necessarily be false - that is to say unrealizable or in contradiction to reality. We call a belief an illusion when a wish-fulfillment is a prominent factor in its motivation, and in doing so we disregard its relation to reality, just as the illusion itself sets no store by verification. [I.e. calling a belief an illusion makes no judgment on whether or not it is true; it is simply stating that it is primarily motivated by the fulfillment of human desires.]
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Let us return to the question of religious doctrines. We can now repeat that all of them are illusions and insusceptible of proof. No one can be compelled to think them true, to believe in them. Some of them are so improbable, so incompatible with everything we have laboriously discovered about the reality of the world, that we may compare them to delusions. Of the reality value of most of them we cannot judge; just as they cannot be proved, so they cannot be refuted. We still know too little to make a critical approach to them. [Once again, remember: 1927. I'm not saying we know everything now; just that we are a lot more knowledgeable about the state of the universe, evolution etc than those of the 1920's.] The riddles of the universe reveal themselves only slowly to our investigation; there are many questions to which science today can give no answer. But scientific work is the only road which can lead us to a knowledge of reality outside ourselves. It is once again merely an illusion to expect anything from intuition and introspection; they can give us nothing but particulars about our own mental life, which are hard to interpret, never any information about the questions which religious doctrine finds it so easy to answer.
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At this point one must expect to meet an objection. "Well then, if even obdurate skeptics admit that the assertions of religion cannot be refuted by reason, why should I not believe in them, since they have so much on their side - tradition, the agreement of mankind, and all the consolations they offer?" Why not, indeed? Just as no one can be forced to believe, so no one can be forced to disbelieve. But do not let us be satisfied with deceiving ourselves that arguments like these take us along the road to correct thinking. Ignorance is ignorance; no right to believe can be derived from it. In other matters no sensible person will behave so irresponsibly or rest content with such feeble grounds for his opinions and for the line he takes. It is only in the highest and most sacred things that he allows himself to do so.
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We have recognized [religious doctrines] as being, in their psychological nature, illusions. This discovery also strongly influences our attitude to the question which must appear to many to be the most important of all [i.e. are they true]. We know approximately at what periods and by what kind of men religious doctrines were created. If in addition we discover the motives which led to this, our attitudes to the problem of religion will undergo a marked displacement. We shall tell ourselves that it would be a very nice if there were a God who created the world and was a benevolent Providence, and if there were a moral order in the universe and an afterlife; but it is a very striking fact that all this is exactly as we are bound to wish it to be. And it would be more remarkable still if our wretched, ignorant and downtrodden ancestors had succeeded in solving all these difficult riddles of the universe.