This post is going to be overly ambitious and overreaching, but isn't that what the blogosphere is all about? Classes started this past week, and I am teaching an upper-level course in philosophy of science. I started the class with the (overly ambitious) question 'What is science?' We had an interesting conversation, and I learned a bit from my students. There was an extended discussion about which particular parts of forensic science really do count as science, and the question 'What is science?' really matters there, because once something is declared accepted science, then it can be admitted into evidence in a court of law.
I had one thought concerning how to distinguish science from the humanities, which I didn't share with my students, but I figured I might try to articulate here. I think it may just be a slightly different way of putting a tired old point, but here goes.
Both the sciences and the humanities seek understanding; both offer explanations of various bits of the world. At a very abstract level, though, the kind of things each tries to explain is different. Obviously, (e.g.) Hamlet is a very different kind of thing than (e.g.) thermometer readings -- I have a more 'formal' difference in mind. When an English professor gives an interpretation of Hamlet, she has (basically: see (3) in next paragraph) ALL the stuff to be explained in front of her: the text is complete, finished. To put the point in terms of evidence instead of explanation, all the data/evidence she can offer for her preferred interpretation of the text is already in. This stands in clear contrast with (most of?) the sciences: new data is constantly being gathered, and new observations need to be explained. If a similar process were occurring in the Hamlet case, a new Act of that play would be produced every week, and various interpretations were shown to be stronger or weaker as new 'data' (i.e. texts) came streaming in.
I think many parts of philosophy are more like the Hamlet case. Take ethics for example. Murder, stealing, lying, etc. are morally wrong. This is in some sense the 'received/ established text' from which the ethicist works: an ethical theory has to explain why those things are wrong. Of course, there are thought-experiments designed to probe various parts of our ethical intuitions, but (1) these often yield contentious/ equivocal results, and (2) more importantly, I only want to say philosophy is more like the Hamlet case, not that it's identical. (Plus, (3) the English professor could potentially get some new information "around the margins" (to put it metaphorically): further historical details of Shakespeare's life, earlier drafts of texts, various facts about the circumstances of production, etc. -- there is clearly a continuum here with absolutely no new data on one end and lots of new data on the other.)
Another philosophical example would be philosophy of arithmetic. We've known that 7+5=12 for some time now, and the philosopher wants to explain how and why we know it's true. We're not getting a whole lot of new 'data' about arithmetic. As a final example, consider the enterprise that falls under the heading of 'interpretation' of various special scientific theories -- an enterprise which is frequently thought of as being particularly close to science. This industry is perhaps most developed in the case of quantum mechanics, but it thrives in general relativity, statistical mechanics, population genetics, etc. as well. On my criterion, these projects (usually) fall much closer to the 'humanities' end of the spectrum: the game here is to take some standard formulation of the theory in question (e.g. of quantum mechanics) and provide an explanation of that theory. The formulation of (e.g.) quantum mechanics is, I think, like the text of Hamlet, insofar as it is taken to exhaust what needs to be explained. However, sometimes this explanation will (either by itself, or with further specification) generate new experimental predictions. When that happens (as in some cases of spontaneous-collapse interpretations of quantum mechanics), then the project becomes more scientific on the above criterion, and less like the humanities.
Perhaps the criterion I've suggested here can partly explain why many (though by no means all!) physicists express exasperation with the philosophy of physics, and consider the enterprise pointless.