Scientism is often defined along these lines:
“Scientism is a term used to refer to belief in the universal applicability of the scientific method and approach, and the view that empirical science constitutes the most authoritative worldview or most valuable part of human learning to the exclusion of other viewpoints.” 
I am amazed that anyone still holds to this view but it appears quite a few people in the New Atheist movement find it quite appealing. What I will record below is a conversation I had recently with someone who holds to this view. The conversation is completely amicable in tone throughout (to the best of my knowledge) but the person holding to scientism could not be moved from his position.
If there is anything you think I could have improved in terms of points made please feel free to say. You will see that I tried to pin him down on one point throughout but it proved impossible for him to see that he needed to give scientific evidence for his opinion that all rational beliefs require scientific evidence. Is there some other way of teasing out this contradiction?
Anyway, here it is. I hope to learn from it and improve next time.
SG = scientism (not to be read as ‘science’!) guy, / ME = me! After a little preamble in what Hosea was or was not apparently saying and how the God of the Bible ordered some pretty evil things it got onto the subject.
SG: “My system of living in this world is governed by evidence (see above). It’s not a belief system, I simply do not and will not believe extraordinary claims without extraordinary evidence. A religion is a system based on non-evidential belief.
If you were going to cross a road, would you check it’s clear? Or would you just shut your eyes and pray it’s clear? One is evidential and the other is belief based. Which one do you do?”
ME: “It’s really no more abstract than you want it to be. You say you demand evidence for rational beliefs but I have pointed to a belief you have (which I have labelled ‘x’ – see above). Now all I’m asking you to do is to provide me with evidence that this belief is rational and true. If you cannot provide evidence then it’s clear you do not always follow the rule of evidence that you claim to. What it means is that, actually, in the most important road crossings of your philosophical journey you not only walk out into the road blindly but you want to take me by the hand and expect me to follow you out into it on nothing but blind faith. I’m sorry but I’m not prepared to do this philosophical road crossing with my eyes shut and my brain turned off. So let me be clear again. The simple question that needs answering is this: You said, “Belief based upon no evidence is irrational, belief based on scientific factual evidence = rational.” I would just like to see evidence that this truth claim is true. At the moment I am just asking for one single piece of evidence but the bad news (which comes later) is that this claim of yours which I have called ‘x’ is what most philosophers would consider to be an extraordinary truth claim because if it were really true it would completely revolutionize modern epistemology. It would also be extraordinary because of the scope of its method. So I could actually ask you for extraordinary, peer-reviewed <wink>, evidence for the veracity of ‘x’ but for now I would just like one piece of evidence please. Understanding this will help you understand why you are offering me a false dichotomy at the end of your post by the way. Do you have anything at all? Many thanks.”
SG: “You’re under the misinterpretation that I expect you to follow my way of life. I couldn’t care less how people live, but all I ask is for that claims people make to be backed up by scientific evidence. Especially as my life is often affected by religious sensibilities.
As I defined rational and irrational then the question answers itself! And as I’m not making claims I don’t have to provide evidence, all I’m doing is asking for evidence. Which there currently isn’t any.
Provide me some evidence to back up your claims, and then I can adapt my world view to reflect the evidence available!!”
ME: “Come on now. The whole reason you’re talking to Christians here is because you care about truth surely? You have made it very clear that you think other people on this thread (Christians) are mistaken and one of your main reasons for thinking this (you claim) is because they don’t have sufficient evidence for you to believe. You clearly DO care how people live their lives since you have also made it clear that some religious people can do harmful things. When you point that out you are not doing so to say you couldn’t care less – you’re pointing to them because you do care. I commend you for that. If you didn’t care about how anyone else lived their lives then you’d be a pretty heartless person and it’s clear you’re not.
Having said that, I have now asked you three times (at least) for evidence and you won’t provide me with any but then you appear to see no hypocrisy whatsoever in demanding that I show you evidence (and, by the way, the type of evidence you’re asking for is what philosophers call a category error). It’s becoming painfully clear you don’t have any evidence whatsoever for your claim that all beliefs that make truth claims should be supported by scientific evidence. Wasn’t it the great philosopher <wink> Hitchens who said “What can be asserted without evidence can be dismissed without evidence.”? Since you have asserted ‘x’ without ANY evidence whatsoever then, according to Hitchens, I am completely within my epistemic rights to dismiss the demands you are putting on me. Why should I show scientific evidence for God if you cannot provide scientific evidence for ‘x’??? Until you can provide evidence for ‘x’ this is going to go nowhere. I just think it’s a real shame you are asking other people to meet demands you yourself cannot and refuse to meet.
This is why I began with moral truths at the beginning of the conversation. You said there were moral truths but the problem is there are no scientific facts which support the contention that there are moral truths. Science can test for pain and it can measure it but what it cannot, and has never been able to do, is show that causing another person pain is morally wrong. Moral imperatives are not the jurisdiction of empirical science. Why do you think these two subjects are found in very different places in academic universities? Is the entire field of academia wrong while you are right? That’s quite an extraordinary claim! Science is out of its jurisdiction on matters of metaethics, metaphysics, epistemology, formal logic etc. and this is also true when it comes to theological questions too. The problem is you have limited your evidence to only the physical. What I am showing you is that you actually don’t demand physical evidence for all the beliefs you hold to which you think are rational. In fact, your underlying theory of knowledge is based on a complete and utter act of blind faith and I have demonstrated that to be the case because you cannot show any evidence for it whatsoever. Nothing – not even one thing.
Now, in contrast to that, there is a lot of evidence for the existence of God but it happens to be evidence based in reason rather than purely on empirical evidence (although the latter can inform the former). So, for example, one very strong piece of evidence is the probabilistic argument based on the fine-tuning of the universe. This evidence is discussed, by the way, in peer-reviewed literature. I think the problem here is that I am actually quite happy to talk about the evidence for the existence of God but you are trying to set the game up so that only one type of evidence can be used and I am explaining to you why that approach utterly fails. The way I showed you it fails is because your belief ‘x’ is completely lacking in the evidence you demand of everyone else.”
SG: “I care about evidence, the whole point of this thread was about trusting what the bible says which I can’t do without evidence. Christians may well be correct, I can’t prove them wrong but I only asked for evidence to prove them right. Muslims may be right, Hindus may be right, even Jedis might be right. Without evidence, their claims are unsubstantiated. I didn’t even ask for a lot of evidence, just a little tiny bit of evidence.
I only care about how people live their lives if it affects me, if it doesn’t then I couldn’t give a monkey what they do. So yes, I am fairly heartless.
Just to make clear, you’d like me to give you evidence that i live my life only ‘believing’ in things that i can prove with evidence right? I’m a little confused about what exactly you are after, put it into simple language please.
You could put me into a laboratory and do as many tests as you like on me, to gather that evidence that I live my life through evidence. That’s not a problem whatsover, I would gladly submit myself to the scientific method. So, in the issue of fairness, let’s also put religion into a laboratory.
At no point have i said that there are moral truths. That’s your words that you are now attributing to me. I have also not said that causing another person pain is morally wrong, it might be morally right. Morals are what society thinks they are. An individual may have their own moral values, but when they go outside of the societal norm, then they are ‘wrong’ for that society. Laws are there to do the judging for us.
Again, you’re confusing philosophical musings with actual evidence. I’ve stated that that is not an acceptable medium for me. It might be for other people, but not for me. My position on evidence is clear. Real, tangible, repeatable scientific evidence. As stated above, at some point in time, a philosopher must test his reasoning with actual experimentation and if they don’t, then all it is a load of words on a page.
If you want an opinion, ask a philosopher, if you want an answer, ask a scientist…the scientist might be right, or wrong, but either way, he’ll have the evidence to show for it.
I’ve also asked for evidence, and none has been forthcoming. So we seem to be at an empasse. I don’t claim to be right, but you do, I only claim to need evidence to believe. So to prove yourself right, just provide me with evidence, should be a simple task..apparently god has been around for ever so there should be some fairly straight forward evidence lying around.”
ME: “Point of clarification please: You are not being clear on the moral issue. You say “At no point have i said that there are moral truths.” which suggests you do not think there are any moral truths but then you state: “Morals are what society thinks they are. An individual may have their own moral values, but when they go outside of the societal norm, then they are ‘wrong’ for that society.” That last quote is clearly a claim to a moral truth. So I would like you to clarify whether you think there are moral truths or there are not. Thanks. When you have done so I will make my ‘closing statement’. I’m really sorry but I don’t have sufficient time to discuss this. I am too busy philosophizing!”
SG: “I’m still waiting on your clarification, so I can answer your question about evidence.”
ME: “I’m not sure what it is exactly you want clarified. Can you please state it exactly like I did in my request for clarification? Thanks.”
SG: “Just to make clear, you’d like me to give you evidence that i live my life only ‘believing’ in things that i can prove with evidence right? I’m a little confused about what exactly you are after, put it into simple language please.”
ME: “I have actually asked this question a number of times and I’m afraid I cannot find any simpler way of asking it than the way I already have. What I want is evidence that ‘x’ is true ( where ‘x’ stands for the belief that “Belief based upon no evidence is irrational, belief based on scientific factual evidence = rational.”). Put another way what I want is scientific evidence that scientific evidence is necessary for rational belief. It’s as easy as that. Do you have any? Yes or no?”
SG: “I see your confusion now, you aren’t accepting my definition of rational and irrational beliefs. There is no answer to your question, as it’s meaningless. Rational belief IS simply the belief that something is true, or false, with evidence. Believing in something that cannot be proved true or false is irrational. It’s simply a definition.
So, I suppose to gather scientific evidence that scientific evidence is necessary for rational belief you would have to question the English language dictionary’s and scientists/linguists about the meaning of the words rational and irrational. What you’re having is a linguistic problem.
Also, I have no idea if there are moral truths, that sounds a bit philosophical to me. If you find one, please subject it the scientific method and let me know how it all pans out. All I know is that various societies have different acceptable moral codes. All I can do is live my life by a simple moral code, don’t be an arsehole. That might be right or wrong, but that’s up to the society I live in to decide. If I break the societies code, I go to jail.”
ME: “Ahhhh – it’s meaningless is it? Of course the claim it’s meaningless is a philosophical one and not a scientific one. (The religious people in here should equally feel free to call “meaningless!” when SG asks for scientific evidence of matters that take place outside of science.) Perhaps you could explain to me then why it is that philosophers of science have been discussing this supposedly meaningless question for the last 80 years? Please feel free to explain to me how it was that Karl Poper did not in fact discredit the logical positivists but how, instead, we should see verificationism as a valid epistemology in modern times. If you can do that you ought to send it for peer review because you will cause a huge upset in the philosophy of science if you’re right. I’m really sorry to have to point this out to you but you are in the middle of a discussion on epistemology here and this is a subject I teach. Now it’s quite possible I am wrong, even though I teach this subject, but the chances are unlikely since this is pretty elementary stuff that A-level students would be expected to grasp. You assert this definition, “Rational belief IS simply the belief that something is true, or false, with evidence.” (You ought to be honest and add the adjective ‘scientific’ before ‘evidence’ since you have already made it clear that this is the type of evidence you are after.) The problem is not linguistic but epistemic. The problem you’re having is you are trying to set up a standard for rational belief (scientific evidence) and when someone rather annoying comes along (like me) and points out that this belief does not have any scientific evidence you try to evade answering and then merely assert the question is meaningless. All I would like to point out to you is that you are setting up a HUGELY contentious theory of knowledge with that definition of what ‘rational belief’ is.
Actually – I did offer you an argument for the existence of God but you must have missed it. I said the argument from the fine-tuning of the universe. That is a very strong probablistic argument in favour of there being a God. So much so that it was one of the reasons that the famous atheist philosopher Anthony Flew abandoned atheism and accepted theism.
You’re quite right that the issue of morality is philosophical. The problem is you want to judge the God of the Bible for what you (wrongly) see as some of his moral actions but at the same time you want us to take you seriously when you admit you don’t actually believe in any moral truths!! Therefore you have no basis upon which to judge anyone else’s moral actions including God. Since you (nor anyone else) has demonstrated that child molestation is morally wrong objectively you cannot say this is wrong for anyone but yourself. You are now playing with a form of moral theory which DOES believe in moral truths and yet somehow you cannot see this. On your thinking it would be okay to sacrifice babies in societies which see this as accetable so long as it’s not viewed as being an ‘arsehole’ in that culture. Sorry but that is cultural relativism and if you really believed that you would have no problems with ancient people’s going to war with each other because they thought that’s what God wanted because that’s just what ancient cultures did. Neither would you care if ancient peoples held bond-servants – that’s just what ancient cultures did. Personally I think you’re backing down on making any ethical truth claims because you don’t want to have to admit there can be truths which do not require scientific evidence in order to be true. I think you know there are some thing which really are wrong in an objective sense but you won’t admit it now because you don’t want to admit it will undermine your other philosophical commitments.
I am sorry that you think philosophy is such a waste of time. It’s a curious position to take at this stage because that’s what you have been trying your hand at for the last day. We have not been talking about scientific matters but philosophical ones. The only time that became a problem was when the philosophy began to show cracks in your theory.
I would strongly encourage you to question your epistemology a lot more. You’re holding onto a very weak epistemology which many call ‘scientism’ and virtually no academics hold to it today. In fact, it’s pretty much ridiculed and laughed at among philosophers of science.
To other theists on this thread I would just beware people who attempt to play this epistemological game. Notice that there were only philosophical arguments here from SG. He is not offering any scientific evidence and yet he’s making a ton of truth claims about what people can believe if they are to be rational.
Unfortunately, as this thread shows, those who hold to scientism don’t actually pass their own criteria. The claim that all beliefs require scientific evidence is itself a belief. If it’s true then science would support it. As you can see from SG’s silence on this matter, there is no scientific evidence. He did not offer one single piece of empirical evidence for this belief of his and yet he requires everone, apart from himself, to have scientific evidence for all of their beliefs before he will take you seriously. This is epistemological nonsense. For those open on such matters I would strongly recommend Keith Ward’s ‘Why there almost certainly is a God’ which shows why this type of thinking is so badly flawed. He shows, very well, how theism is far more rational than atheism in my opinion.”
SG: “The problem is you won’t accept that I require scientific evidence to believe something, and I won’t accept anything less than scientific evidence. It’s that simple. Good luck with your world view, and good luck with mine.
I don’t have a problem with ancient peoples going to war with other ancient peoples because a ‘god’ told them to, actually, I rather like the idea. War is a good method of culling an excessive population. I do have a problem with modern day peoples believing unprovable things when those beliefs impact my day to day life (religion in politics etc).
Again, what I hold isn’t a belief in the religious sense, it’s just a need to have scientific evidence before I believe something. How hard is that for you to understand? You seem to revel in not understanding that. It’s not a belief system, a religion, or even a life choice, it’s just simple hard facts. I ask that religious people examine the facts and evidence of their religion, that’s it. Are they wrong, I think so, can I prove it..no. Therefore I can’t say for certain that they are wrong.
Again, books on philosophy aren’t proof. No matter how hard you try to push that, it still doesn’t make it fact. Just because something is discussed, doesn’t make it true, no matter how long or convoluted the discussion is. Facts aren’t a democracy. I suggest you put down your books, and start gathering real, tangible evidence and once you have some, please bring it to me. I will gladly look at it, and immediately change my world view if it proves correct. That’s the big difference between a religous person and myself, I’m 100% open to change. 
Right from the start I asked for evidence, none as been forth coming. Therefore I can keep my view that the bible isn’t correct as no evidence to the contrary has been given.”
ME: “No worries. Since you offered me no scientific reason to accept your theory of knowledge I will feel free to ignore it (just like academics tend to). Thanks.”
I then offered a couple of videos which I will place below and that was pretty much it.
Did I miss making a point at a key moment of the conversation? There is a small degree of frustration for me at the end since I cannot believe anyone really thinks they are still doing science when they are clearly doing epistemology but I’m sure almost anyone who has done any philosophy would have been pulling their hair out at this stage too.
 Wikipedia ‘scientism’.
 Peter Boghossian would have been proud!
For those who wish to hear more on the inadequacy of scientism, John Lennox explains in part of this superb lecture (the part about scientism is from c.10 minutes in):
Here is a nice reply by Fr. Robert Barron:
A lecture on scientism by Prof. Mikael Stenmark:
J.P. Moreland on scientism vs Christianity: