In my Pillars of Science series, I enumerated six aspects of Science that help explain why it works so well.
It should be clear from my analysis that the characteristics of Science are quite flexible. All of the criteria are matters of degree, so that they are met more strongly by some fields of study than by others. Because of this fuzziness, we should expect to find borderline sciences, such as Economics, Anthropology, Psychology, and other social sciences. It is both futile and unnecessary to try to come up with a criterion to draw an exact line between science and non-science. In other words, the question of what counts as Science cannot itself be resolved with scientific precision, and is therefore not a scientific question.
This doesn't bother me too much because my parents are linguists. So when I was growing up, they made sure I was aware that concepts are defined by their centers, not their boundaries. For example, if I say the word "chair", then what pops into your mind is a thing with four legs at the dinner table. You might admit under interrogation that a "beanbag chair" is also a chair, but it's hardly the first thing you'll think of. Concepts can be useful even when they're a bit fuzzy at their boundaries.
Despite their flexibility, the criteria are sufficiently strict that many things don't qualify. I don't just mean pseudo-sciences such as astrology or homeopathic medicine, but genuine evidence-based fields of knowledge (“sciences” in the archaic sense of the word) which aren't scientific in the modern sense, because they only satisfy some of the criteria.
For example, History and and Courts of Law, despite their empirical character, deal mostly with unique and unrepeatable events. So they fail the repeatability prong of Pillar I. Both of these fields are based primarily on testimony of witnesses, although Law Court fact-finding has much stricter rules about admissibility of evidence. Since much of their subject matter can't be defined with quantitative precision, they don't do terribly well on Pillar IV either. Academics in History do have a truth-seeking community similar in kind to the Sciences. But in Law Courts, the role of ethics, community, and authority is completely different.
This does not mean that these fields should be held in contempt; their methods are sometimes capable of establishing specific facts with a very high degree of certainty, “beyond a reasonable doubt” as the saying goes. They simply lack the particular methodology of science, which has a proven track record of almost routinely proving astonishing facts about the world, to a degree that ends rational opposition. If you try to increase certainty by imposing a “scientific” approach on a subject that isn't suited for it, you risk generating a pseudo-science which jingles the jargon of science while missing its core value: self-correction through rigorous testing of ideas.
Philosophy is nonscientific for a different reason than the empirical humanities. While many philosophers strongly value elegance and precision of ideas, typical disputes between philosophers are not very amenable to empirical testing. That doesn't mean that observation plays no role. But the way philosophers typically make arguments, they also rely on controversial background assumptions, which can't be definitively settled just by looking at the world.
If, despite the potential for controversy, the argument for the position is sufficiently convincing, this can still establish the philosophical position with great certainty. In fact, unless the skeptical thesis that no knowledge is reliable could be refuted with near certainty, the result would be that no field of inquiry could produce near certainty. This potential for certainty does not change the fact that Philosophy operates by a different methodology, which on average does not resolve controversies as easily as the methods of Science or even History do.
For this reason a philosophical thesis based on Science will usually have the degree of certainty associated with Philosophy, not that associated with Science. A chain of reasoning is only as strong as its weakest link. So a philosophical argument based on Science should not necessarily trump, e.g. a strong historical argument, simply because Science is normally more reliable than History.
So how do we fit ideas from different fields together? In a future post, I'll discuss Bayes' Theorem, which is a flexible way to think about all different kinds of evidence-based reasoning, without making specific assumptions about the sorts of evidence we can include.