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I migranti di Fahrenheit migrano ancora… (IV inbloggazione)

Valutazione probabilistica del cristianesimo

Ci sono buoni motivi per ritenere che Dio sia la risposta più adeguata alle “domande ultime”. Ma quale Dio? Forse il Dio cristiano?

Richard Swinburne nel suo “Was Jesus God?” affronta la questione partendo dall’assunto teista:

… I assume in this book that, on the basis of evidence such as the general character of the natural world or a person’s own religious experience, there is a moderate probability that there is a God of the kind worshipped by Christians, Jews, and Muslims…

E’ necessario comprendere bene la logica per giungere a conclusioni razionali su questi temi. Potremmo chiamarla la “logica del detective”. Dapprima, bisogna valutare le ragioni a priori, ovvero: come ci aspettiamo che un Dio si presenti a noi? Quali caratteristiche prevediamo che debba avere? Questa prima fase è una riflessione a tavolino, meramente astratta. Poi, si va a verificare i fatti: cosa è successo nella storia? In quale contesto emerge la maggiore compatibilità con le nostre attese?

… I shall set out the central theological doctrines of Christianity (that is, doctrines about the nature and actions of God), and give a priori reasons for believing them to be true. By ‘a priori reasons’ I mean reasons arising from the very nature of God… I shall argue that, given the moderate probability on other evidence that there is a God and given these a priori reasons, the historical evidence about the life and Resurrection of Jesus and the subsequent teaching of the Church makes it very probable that these doctrines are true. This historical evidence provides what I shall call ‘a posteriori’ reasons

Chi conosce il gioco probabilistico sa che anche una modesta probabilità a priori aumenta notevolmente se corroborata da fatti registrati a posteriori.

Tanto per essere chiari: se l’esame del DNA rende Bossetti colpevole con una certa probabilità (a priori),  il fatto che una telecamera abbia (forse) ripreso il suo furgone vicino alla palestra il giorno del delitto (fatto a posteriori), rende la colpevolezza praticamente certa poiché la probabilità a priori, per quanto modesta, esplode di fronte ad una conferma, per quanto incerta.

Facciamo un altro esempio. E’ stato uccisa la zia ricchissima e il commissario sospetta dell’unico erede: il nipote scapestrato. Si tratta però di un sospetto a tavolino, non sappiamo niente di lui se non che esiste un movente, dobbiamo ancora iniziare le indagini cosicché possiamo ben definire “modeste” le probabilità della sua colpevolezza. Poi però le indagini ci fanno scoprire che il nipote “tramava”, che era in bolletta e bisognoso di soldi, addirittura che qualcuno l’ha visto entrare nella villa della zia il giorno dell’omicidio. E’ chiaro che questi “fatti” a posteriori sono talmente compatibili con la probabilità a priori che fanno esplodere la probabilità che sia lui il colpevole.

Allo stesso modo: ragionando sui fatti della natura e dell’uomo giungiamo alla conclusione che forse esiste un Dio infinitamente buono e desideroso di condividere da vicino le sofferenze della sua creatura. Si tratta di probabilità minime anche se importanti rispetto alle spiegazioni alternative. Poi veniamo a sapere che nella storia, a quanto pare, si è manifestato un Dio in forma incarnata. Ora, anche se la probabilità a priori è modesta e la probabilità che il “Dio Incarnato” si sia realmente manifestato nella storia non è una certezza ma solo una testimonianza con certe caratteristiche, la combinazione tra i due eventi fa schizzare le probabilità a posteriori ad un livello ragguardevole.

Ripetiamo allora quel che intendiamo per “Dio”.   

… What I mean by my claim that there is a God is that there is (at least) one divine person, who is essentially omnipotent, omniscient, perfectly free, and eternal… A person is a being who has (or, when fully developed, will have) powers (to perform intentional actions, that is, actions which he or she means to do), beliefs, and free will… God is supposed to be unlimited in his power; that is, God is omnipotent, he can do any action. He can make a physical universe exist, or move the stars, or sustain or abolish the physical causes… He cannot do a logically impossible action, that is, an action which cannot be described without contradiction… God is supposed to be unlimited in his beliefs; that is, God is omniscient; he has all true beliefs about everything… God is supposed to be a perfectly free person in the sense of one whose choices are in no way limited by, that is, influenced by, irrational forces. He only desires to do an action in so far as he sees a reason for doing it, that is, in so far as he believes that it is a good action to do… The best action is that which there is most reason to do. Although we humans are not in general perfectly free, we are sometimes in this situation where we are not influenced by irrational forces…Sometimes, however, a perfectly free being will have a choice between two or more possible actions, only one of which he can do, when he believes that none of these actions are better than the other actions. There is, he believes, no best action but there are two or more equal best actions… In so far as someone believes that an action is good, they will have a desire to do it. Such desires are rational desires…

La creatura di Dio è l’uomo. Il dono maggiore che Dio ha fatto all’uomo è la libertà, anche se si tratta di una libertà con dei limiti:

… But humans are sometimes subject to irrational desires, that is, desires to do bad actions, or desires to do actions less good than a best action, which are stronger than the desire to do the best action…

La libertà dell’uomo è un dono talmente radicale che sembrerebbe limitare anche l’onniscienza divina

… Suppose that I have a choice between mowing the lawn and watching the television; I believe that it would be the best action to mow the lawn but I am subject to a stronger irrational desire to watch the television. What I will do depends on my free choice at that time. If you believe beforehand that I will watch the television, I have it in my power (by mowing the lawn) to bring it about that your belief proved false. Certainly if you know that my desire to watch the television is a strong one, you may rightly think it probable that I will watch the television, but you cannot be certain. It seems to follow that not even God can have an infallible true belief and so infallible knowledge about whether I will watch the television or mow the lawn. Generally it looks as if it is not logically possible for God to know infallibly beforehand what a free agent will do in such circumstances. But since God is omnipotent, it is only because he permits this that we have free will and are sometimes situated in circumstances where we are subject to irrational desires or have a choice between what we believe to be equal best actions…

Comprendere che la libertà dell’uomo puo’ spiazzare anche Dio ci aiuta a comprendere perché talvolta Dio decide di intervenire nella storia, magari esaudendo certe nostre preghiere che evidentemente erano per lui “inattese”. Un motivo per cui Dio non programma necessariamente il migliore dei mondi possibili è il fatto che teoricamente possono esistere mondi moralmente equivalenti (che cessano però di essere tali nel tempo per effetto della libera ed imprevedibile azione umana).

… So although not influenced by irrational forces, God cannot always do the best action. He cannot do this when two or more possible actions are equal best; or where—as in the example just given—there is no best or equal best possible action; and he has then to exercise his choice between the actions in an arbitrary way…

Per quanto riguarda la morale, distinguiamo due tipi di azioni:

… Good actions can be divided into those that are obligatory (or duties), and those that go beyond obligation and which we call supererogatory. I am obliged (it is my duty) to pay my debts, but not to give my life to save that of a comrade—supremely, ‘supererogatorily’ good though it is that I should do so. To fail to fulfil an obligation is to do something wrong. A person is in some way at fault for doing what is wrong, and if he believes that he is doing wrong, he is blameworthy for doing it… since it is always a best action to fulfil an obligation when one has no conflicting obligations, God will fulfil all his obligations… It follows from the argument of the last few pages that we must understand God being perfectly good… God is also a source of moral obligation in that his command to us to do some action makes it obligatory for us to do that action when it would not otherwise be obligatory…

Sull’eternità divina Swinburne fa precisazioni importanti:

… God is eternal. But this has been understood in two different senses: either as the claim that God is timeless (he does not exist in time, or at any rate in our time) or as the claim that God is everlasting (he existed at every moment of past time, exists now, and will exist at every moment of future time). In my opinion the timeless view is incompatible with everything else that religious believers have wanted to say about God. For example, it does seem strongly that God being omniscient entails that he hears the prayers of humans at the same time as they utter them; yet on the timeless view God does not exist at the same time as (simultaneously with) any moment on our timescale. For this and other reasons I shall in future understand God being eternal as God being everlasting; though it might be possible to re-express much of the rest of what I have to say on the assumption that God is timeless rather than everlasting. Being everlasting, God is unlimited in the time during which he exists…

Dio crea e sostiene l’universo. Si badi bene a quel “sostiene”: anche un universo eterno richiederebbe la sua esistenza.

… But God has other properties which are non-essential (accidental or contingent); he has these accidental properties because he chooses to have them. Among these properties is being creator and sustainer of the universe…

Anche se la cosa è disputata, possiamo affermare che Dio non abbia una sua essenza identitaria poiché le altre sue proprietà bastano a definirlo con precisione: non puo’ esistere “un altro Dio”! Dalla sua definizione discende la sua unicità. Perché allora fare cio’ che non è necessario? Per semplicità è meglio allora non attribuire a Dio una proprietà superflua.

… Does God have thisness? Fairly few philosophers and theologians have faced this question, but those who have claim in effect that God does not have thisness. For example, Augustine (the great theologian who was a bishop in North Africa in the fourth century AD) denied that God is properly called a ‘substance’ that ‘has’ properties. God, Augustine claimed, is more properly called an ‘essence’ because he ‘is’ his properties… For reasons of a kind which I shall give later in this chapter, I think that Augustine’s view is correct: if there is a God, God does not have thisness…

Chiediamoci ora perché crediamo in Dio?

Ci sono diverse vie per giungere alla fede, alcuni hanno avuto un’ esperienza religiosa diretta.

… Some people have deep private ‘religious’ experiences, as it seems to them, of the presence of God. Others believe that there is a God on the basis of testimony; that is, because their parents or teachers or priest tell them that there is a God, and they think their parents or whoever are knowledgeable and trustworthy. It seems to me that religious experience provides a good reason for believing—so long as that experience is overwhelming, and you don’t know of any strong objections to the existence of God…

Altri hanno ricevuto la testimonianza di persone che ritengono particolarmente affidabili.

Ma in un mondo sempre più secolarizzato le esperienze religiose dirette sono difficili da realizzare – il contesto è sempre meno adatto – così come si rarefanno le testimonianze carismatiche. E’ per questo che oggi  la ragione e la teologia naturale si pongono come le vie maestre alla fede.

… But I think that very few people have overwhelming religious experiences, and in the modern world most people come into contact not merely with those who tell them that there is a God but also with those who tell them that there is no God, and most people are aware of strong objections to the existence of God… So I think that most people in the modern world need to have their experiences or the testimony of others reinforced by reasons to suppose that the objections to the existence of God do not work. But instead or as well as such reasons, they also need a positive argument for the existence of God which starts from very obvious observable data…

E qui torniamo a quanto dicevamo prima: un buon investigatore privilegia l’ipotesi più probabile che coincide – coeteris paribus – con quella più semplice:

… Suppose that there has been a burglary: money has been stolen from a safe. The detective puts forward the hypothesis, to explain the money having been stolen, that John robbed the safe. If John did rob the safe, it would be quite probable that his fingerprints would be on the safe, that someone might report having seen him near the scene of the crime at the time it was committed, and that money of the amount stolen might be found in his house. These are data to be expected with some modest degree of probability if John robbed the safe, and much less to be expected if he did not rob the safe; they therefore constitute positive evidence, evidence favouring the hypothesis. On the other hand, if John robbed the safe, it would be most unexpected (it would be most improbable) that many people would report seeing him in a foreign country at the time of the burglary. Such reports would constitute negative evidence, evidence counting strongly against the hypothesis. I shall call evidence of either kind posterior evidence, the consequences to be expected or not to be expected if the hypothesis were true. In so far as a hypothesis makes it probable that we would find all the data we find, and in so far as it would be improbable that we would find these data if the hypothesis were false, that increases the probability of the hypothesis… But a hypothesis is only rendered probable by data in so far as it is simple… A hypothesis is simple in so far as it postulates few substances and simply describable properties… The detective’s original hypothesis postulates only one substance (John) doing one thing (robbing the safe) which leads us to expect the data… But as well as the posterior evidence of the kind which I illustrated, there may be background evidence, or prior evidence: evidence which is not a (probable) consequence of the truth or falsity of the hypothesis in question, but comes from an area outside the scope of that hypothesis. We may have evidence about what John has done on other occasions, for example, that he has often robbed safes in the past… Prior evidence: evidence which is not a (probable) consequence of the truth or falsity of the hypothesis in question, but comes from an area outside the scope of that hypothesis…

I criteri con cui selezioniamo “l’ipotesi di Dio” sono i medesimi di quelli che utilizziamo quando selezioniamo le ipotesi scientifiche più promettenti:

… The criteria for assessing the detective’s hypothesis apply generally to assessing hypotheses proposed by scientists or historians… The data (the posterior evidence for theism) to which arguments of natural theology typically appeal include the most general features of the universe: that every particle of matter behaves in exactly the same lawlike way as every other particle (obeys the same ‘laws of nature’, for example, Newton’s law of gravity); that the initial state of the universe (the Big Bang) and the laws of nature are such as to bring about the eventual existence (some 13 billion years later) of human beings; and that these humans are conscious beings (have a mental life of thought, feeling, and choice)…

L’ipotesi di Dio è molto semplice:

… theism is a very simple hypothesis. It postulates the existence of one entity (one god, not many gods), with very few very simply describable properties. A person with no limits to his power, knowledge, freedom, and life is the simplest kind of person there could be… theism is a very simple hypothesis. … Infinite power is power with zero limits. Infinite knowledge is knowledge with zero limits because it involves no limit (except one imposed by logic) to the number of well-justified true beliefs. Perfect freedom means that the person’s choices are unlimited by irrational desires. Eternity means no temporal limit to life. And God being ontologically necessary, meaning that there are no others on whom he depends, obviously fits well with his other properties. It is also simpler to suppose that God has these properties essentially, for that makes God a more unified being… it is simpler to suppose that God is what he is solely in virtue of his essential properties; that is, he has no underlying ‘thisness’—for that is a more economical supposition… If God does not have thisness, any God in charge of the universe would be the same God as any God in charge of the universe…

Alcuni fatti sembrano confutare l’esistenza di Dio. Per esempio: perché esiste il male?

Tuttavia, si tratta di obiezioni superabili se mettiamo al centro la libertà umana e l’unicità di ogni creatura: la libertà deve essere necessariamente anche libertà di fare il male. L’unicità richiede che ciascuno di noi – per sottoporsi ad un giudizio equo – affronti prove differenti, e alcune di queste prove implicano di testare la nostra libera reazione di fronte al male naturale.

…arguments against the existence of God from the fact that there is much pain and other suffering in the world do not work… God wants to give us deep responsibility for ourselves and each other. And he wants us to choose to exercise our responsibility in the right way. So he takes a big risk with us… So gradually over time we change the desires which influence us, and we may eventually form either a very good character or a very bad character… Humans need the pain and disability caused by disease and old age if we are to have the opportunity to choose freely whether to be patient and cheerful, or to be gloomy and resentful, in the face of our own suffering; and the opportunity to choose freely to show or not to show compassion to others who suffer, and to give or not to give our time and money to helping them… God has the right to impose on some of us bad things…

Ma perché mai Dio avrebbe il diritto di “testarci” mettendoci nei guai per vedere come ce la caviamo? In alcuni casi si arriva a far soffrire gli altri per offrire una possibilità a terzi di dimostrare il loro coraggio. E qui viene buona l’analogia col genitore:

Parents have a very limited right to allow their children to suffer for the sake of some good to others. They have the right to send a daughter to a neighbourhood school which she will not enjoy very much, in order to cement community relations… It is nevertheless a great privilege to be of use to someone else, not just by what you choose to do but by what you are allowed to suffer… But if there are any humans in whose lives (not as a consequence of their own choices) the bad exceeds the good, God has an obligation to give to those humans at least a limited life after death in which the good exceeds the bad; and in his omnipotence he can and must do this…

Lo scopo del libro è quello di verificare la coincidenza di talune dottrine cristiane con le affermazioni  che si possono fare a tavolino ragionando sull’ipotesi teistica, che si assume essere la probabile tra quelle a nostra disposizione per spiegare l’universo. In questo caso le probabilità del teismo coincidono con le probabilità a priori da aggiornare analizzando l’evento cristiano. Otterremo così delle probabilità a posteriori da confrontare con quelle calcolate su ipotesi alternative. Nel caso in cui la dottrina e la storia del cristianesimo risultassero “attese”, questo innalzerebbe in modo consistente le probabilità a posteriori del cristianesimo.

… The earlier evidence which formed the posterior evidence for bare theism forms the prior evidence for Christian theism… I contrast this prior evidence with the posterior evidence for Christian theism, which is the historical evidence about Jesus and the subsequent Christian Church. (This provides a posteriori reasons for believing Christian theism to be true.)… In so far as the historical evidence is to be expected if Christian theism is true and not otherwise (that is, in so far as Christian theism makes it probable that this historical evidence will occur, when it would not be probable otherwise), that will raise the probability of Christian theism well above its prior probability, and give it what is called its posterior probability, its probability on the total available evidence…

Le dottrine principali (comuni a tutti i cristiani) prese in considerazione sono contenute nel Credo niceano. Ne cito tre a titolo di esempio: 1) resurrezione di Gesù 2) incarnazione di Gesù 3) Trinità. C’è poi l’insegnamento morale, anche quello va passato al vaglio.

– continua –

santità

 

 

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