One often hears the word 'scientism'. I've never really liked the term, since it fairly clearly is misleading as to what it is supposed to describe. The term makes it sound like it has some foundation in actual scientific work, but in fact people use it to describe something that has no such foundation. What people are really complaining about when they talk about 'scientism' is science-fictionism, scientifictionism. That is, the problem is not the appeal to science; the problem is that there is no appeal to science but an appeal to results one's speculative vision of science will eventually reach. This way of drawing conclusions differs from that of someone who is merely trying to describe actual scientific results we have in hand, or even someone who holds the same conclusion but does so because they have considered the alternatives and think them incoherent. So, for instance, while you will find physicalists who are physicalists because they have considered the alternatives and have concluded that they involve serious logical problems, most physicalists are not so reflective, and instead believe in physicalism because of a vision of physics not yet in hand which will offer a complete account of mind and all. Drawing conclusions about the world on the basis of a science we don't have is science fiction.
Now, again, the conclusion itself is not necessarily the problem. One could hold the very same conclusion for reasons that have nothing to do with fictional dreams about what science may be someday. Someone who is a determinist, denying free will, because they think they have a solid argument in hand that 'free will' is an incoherent idea is not believing something on the basis of a science-fiction story they've told themselves; but someone who is a determinist because they think that in the future psychology will be rigorously reducible to physics is. Likewise, the conclusion could even be approximately true, in the way science fiction sometimes is; this has no bearing one way or another. Likewise, one might think a speculatively drawn conclusion really worth thinking about, but have the sense to recognize it as speculative.
The issue is rather the way the conclusion is drawn. The scientifictionist wants us to believe something because science is amazing and so can overcome whatever obstacles we may think exist to the truth of their conclusion. The problem, of course, is that he completely overlooks the possibility that science is amazing and so can discover that the scientifictionist's own conclusion is incorrect. There is nothing holding up his conclusion but his own imagination; and that he is right in his own imagination is neither particularly interesting nor particularly informative for anyone else.
Since scientifictionism is not primarily a matter of the conclusions drawn but how they are drawn, scientifictionists come in many different stripes. Some are reductivists, some eliminative materialists, some nonreductive physicalists, some determinists, some intelligent design theorists. There are others. And, of course, you can find examples of people holding each of these positions for other than scientifictionist reasons. What they share in common is not their conclusions but the fact that they are really just offering the framework of a science fiction story. Such things are quite nice and worthy of serious thought -- as science fiction, not as reasons to believe.
