Philosophy takes place within a social, political, and intellectual context. There is a surrounding culture or environment. Religion, morality, the arts, the sciences, war, peace, a general optimism or pessimism—all these factors impinge on the way philosophy is practiced during a particular historical period. The factors can vary over time, causing philosophy to vary over time (also place). A given period may be preoccupied with rival political systems (ancient Greece in Plato’s time) or with the advent of natural science (seventeenth century Europe) or with the arts and architecture (Renaissance Italy) or with war and religion (early twentieth century Europe) or with populism and social media (today almost everywhere). Philosophy is apt to be shaped by these preoccupations, leading us to suppose that philosophy is historically constituted: it is the intellectual treatment of prevailing cultural formations. Philosophy is the philosophy of this or that (non-philosophical) area of human endeavor, an essentially second-order activity, so that its content is fixed by the prevailing cultural concerns. It is, in a broad sense, political, using that word widely to connote societal movements and developments: it is politically engaged, politically formed. This is not true of other intellectual domains: physics and mathematics, say, are socially detached, apolitical. They have their own separate identity that transcends passing cultural moments; they occur in history but they are not of history. But philosophy, it may be felt, is inherently historical, and hence political in the broad sense. It feeds off history, societal context, and the affairs of the moment. It was different in ancient times and it may be different in the future; it may even be unrecognizable in the distant future. Philosophy is changeable and fluid, without any solid constant core—like literature, or politics itself.
I think this view is profoundly mistaken, though I understand its appeal. Philosophy consists of a fixed set of core problems that are invariant over time and social context. These problems have a specific identity that is quite independent of political factors. A typical philosophy curriculum gives a fair sense of them: problems of metaphysics, epistemology, ethics, mind, language, logic, aesthetics, etc. I need not list these problems—we are familiar with them. They often take the form “What is X?” where X might be causality, time, space, knowledge, justification, the right, consciousness, reference, necessity, beauty, etc. It is notoriously difficult to say what unites these many problems under the heading “philosophy”, but we know it when we see it: the problems strike us as peculiarly intractable, debatable, puzzling, confusing, and fascinating. We call this quality philosophical, as in “That’s a philosophical question” or “Now you are getting philosophical”. The quality does not normally belong to other types of questions—questions that are factual or empirical or straightforwardly answerable. We are reduced to saying that philosophy is like jazz—you know it when you hear it. It is not easy to define the scope of other disciplines either, but at least we have short adjectives that give some sense of what the subject is all about. What is physics about? Well, there are many branches of physics, quite heterogeneous, but we can say (though not very illuminatingly) that they all concern the physical. In psychology, too, we find considerable heterogeneity and many branches, but at least we can say that they all concern the mental—even though that term covers a wide variety of phenomena. But in philosophy we seem stuck with the adjective philosophical, which is especially unhelpful. We know the quality when we see it, but we find it hard to articulate it with any clarity (it is that quality—whatever it is–that gives rise to a certain sort of intellectual cramp or perplexity or bafflement). I don’t think this difficulty undermines the legitimacy of the subject—after all, philosophy includes pretty much every area of human endeavor—but it makes the question of the nature of philosophy hard to answer. We can say that philosophy is concerned with concepts, but that risks misunderstanding and is surely too narrow as it stands—and isn’t psychology also concerned with concepts? In what way is philosophy concerned with concepts, and to what end? What is the nature of its questions, and what method does it use to answer them?  We can reply that it is concerned with concepts philosophically, or that it deals with philosophical questions about concepts, or that it uses the philosophical method to analyze concepts: but this leaves us where we started. It isn’t false to say that philosophy is concerned with concepts—in fact, it is perfectly correct—but it doesn’t give us much to go on. We do better to list the standard philosophical problems and say, “That is what philosophy is”. If you want to know what it is for a question to be philosophical, then acquaint yourself with some philosophical problems: then it will become manifest to you. These problems constitute the subject matter of what we call “philosophy”, and they are independent of time and context. They are self-standing, specific, and timeless. They transcend history.
How do the problems of philosophy relate to science? I wish to say two things about this: (a) the problems of philosophy are not scientific problems, or pre-scientific problems, and (b) philosophy is itself a science, but of a special sort. With respect to (a) it has often been maintained that philosophy is “continuous with science”—that it does not essentially differ from the accepted sciences. Perhaps it integrates or summarizes the sciences, or perhaps it is just more general but in the same line of business. One often hears it said, particularly by scientists, but not only by them, that the history of philosophy is the history of parts of philosophy splitting off and becoming real sciences—as physics split off from “natural philosophy” to become the science it is, and as psychology is still in the process of doing. This is taken to be a good and necessary thing, as if the splitting off were a step towards intellectual respectability after a shady past. Thus it is assumed that all of philosophy will eventually metamorphose into science, and that what does not achieve this happy transition will be left to wither in peace. I think this view is completely wrong: philosophy is not continuous with science and its history is not a process of peeling off to become science. For philosophy consists of a distinctive set of peculiarly philosophical problems that are independent of cultural context, which includes science. The problem of skepticism, say, is not a scientific problem, and will never become one; nor is the mind-body problem a scientific problem; nor are the problems of ethics; and so on. Philosophy is just a different kind of subject—being concerned with problems of a philosophicalnature. It characteristically wants to know what something is (essentially is), or how a problematic phenomenon is possible (consciousness, free will, a priori knowledge), or how one thing is consistent with another (knowledge with fallibility, contingency with determinism, emergence with novelty). In a very broad sense, philosophy is concerned with logical questions—questions of definition, essence, entailment, and how things fit coherently together. It is about constructing a logically satisfying worldview. It aims to make things rationally intelligible (as opposed to discovering particular facts). It uses reason to make sense of things, and reason is an exercise of the logical faculties (not the sensory faculties). Philosophy is about the logical structure of reality.
Regarding philosophy in this way, as a logical enterprise, opens the door for a salutary extension of the word “science”. Philosophy is a science—a logical science, a formal science. I like to call it “ontical science” by analogy with “physical science”: it is the general science of being. It is the science of what things essentially are, what their constitutive nature is; this is why definition looms so large in philosophy. What exactly is knowledge, free will, consciousness, moral goodness, necessity, causation, beauty, truth, the self, rationality, and so on? Philosophy approaches such questions in a scientific spirit, employing reason, careful reflection, logical deduction, and theory construction. It is not poetry or mysticism or propaganda or politics. Its results are checkable, rationally debatable, and intended to state the objective truth. One of its methods is the thought experiment—imagining possible states of affairs and asking how a given concept would apply in them. This is a genuine type of experiment—a procedure in which the outcome is not prejudged and which can be repeated by others. For example, imagine a situation in which someone has a true belief but no justification for that belief: does this person have knowledge? We can perform such experiments and obtain inter-subjectively verifiable results (which is not to say they are infallible—but what experiment is?). They can even be described as “empirical” in the sense that we can learn from the experience of performing them. I have discussed this in detail elsewhere and will not repeat what I have already said.  The key (and encouraging) point is that there is nothing to prevent us from describing philosophy as a science, though a science with its own distinctive character. It is a science in its own right and will not devolve into another type of science: it is a sui generis science. Just as the formal science of mathematics will never turn into physics or psychology, so the “ontical science” of philosophy will never turn into any other science. Its problems are what they are and not some other thing. Thus we can say that the ahistorical subject of philosophy—that core of timeless philosophical problems—is a science in its own right. It is not “continuous” with other sciences in the sense of being just like them, or parasitic on them; rather, it is a science that belongs alongside the other sciences, an equal member of the club. We have the sciences of physics, chemistry, biology, psychology—and philosophy. Philosophy is “being-in-general science” (an Aristotelian conception).
To describe philosophy as a science raises expectations of progress analogous to the progress obtained by the other sciences. But does philosophy make this kind of progress? Doesn’t its lack of comparable progress undermine its title to quality as a science? My reply is that these expectations are prompted more by conversational implicature than by logical (semantic) implication. Strictly speaking, the question of scientific status and the question of scientific progress are logically independent: the former does not entail the latter. Non-science can make progress and science can fail to make progress. You can make progress writing a novel or a biography without those things being forms of science, and some parts of science can be mired in controversy and resistant to progress (quantum theory, the origin of life, the psychology of creativity). Some sciences are simply more difficult than others; it is really a complete fluke that astronomy has made the progress it has (fortunately light travels very fast and preserves information). The question is controversial but I would say that philosophy has made impressive progress over the last 2,000 years, though large parts of it have not made the kind of progress we see in the other sciences. The reasons for this are debatable, but I think we can agree that central philosophical problems have not yielded to solution in the way many scientific problems have. One possible view is that philosophy bumps up against the limits of human intelligence—that it consists of “mysteries” not “problems”.  In philosophy we are mapping the outer limits of our intellectual capacity, which must be finite and specific if we are evolved creatures with limited brains (like all other creatures on earth). We are using our science-forming capacities to do philosophy, as we do in the other sciences (empirical and formal), but these capacities have their necessary inbuilt limits—and philosophical problems tax these limits. This is no detriment to the idea that philosophy is a type of science; it is just an especially difficult type of science. If we imagine beings intellectually inferior to us trying to do physics, we can envisage that they are recognizably capable of scientific thought but their talents do not match our own—maybe they can get as far as Newtonian physics but then their brain engine runs out of gas. Just so there might be beings that can handle philosophical problems better than we can, but that doesn’t mean that we aren’t really doing philosophy. Progress is a matter of contingent intellectual capacity; being a science is a matter of the intrinsic nature of the questions. Philosophy, considered as a set of questions, qualifies as a science, even though our capacities in doing it are less than stellar. Or maybe every possible thinker would stumble over philosophical questions, given their intrinsic character; but that would just show that philosophy is a very difficult science. After all, Newton’s intellect was defeated by the nature of the gravitational force, as he admitted, but that doesn’t mean Newtonian physics isn’t really science. In fact, I would say that nearly all science is confronted by deep mysteries, some possibly terminal, but they can still describe themselves as science. Not all science is successful science.
Philosophy is particularly concerned to get clear about things, so clarification is a central part of its mandate. It tries to make sense of things by clarifying them. It aims to render the world intelligible. The italicized words here are all redolent of language: words can be clarified, sentences can make sense (or not), and language is intelligible (though not always). This suggests that meaning is central to the philosophical enterprise: the philosopher is a student of meaning. We can understand this in two ways: the meaning of life, and the meaning of language. Both have been thought to come within the purview of philosophy, and properly so. It has even been maintained that philosophy is exclusively concerned with linguistic meaning–that its sole job is to clarify the meaning of words and sentences. “What does it all mean?” might be thought to encapsulate the philosophical quest.  The narrow interpretation of this is that philosophy asks what words mean. This is not as narrow as it doubtless sounds, since word meaning brings in extra-linguistic reality, but so formulated the question leaves a lot out. I want to suggest, however, that it captures the essence of the matter: for philosophy is certainly concerned with intelligibility—though not only of language. Philosophy is concerned with the intelligibility of the world. It tries to make intelligible sense of the world by clarifying it. We want, for example, to understand the nature of causation (the thing not the word), so we try to clarify what it involves; perhaps it appears unintelligible to us and we need to restore it to intelligibility (as some have thought regarding causal necessity). We want to clarify its logic (essence, nature) so that it can meet our standards of intelligibility. We can do this by analyzing the word, or we can focus on the thing itself and try to discern its intelligible nature. Either way we are trying to achieve clarity by demonstrating intelligibility. The human mind wants to make sense of things, and philosophy is the tool for achieving this. So philosophy is a sense-making science—a science that aims at clarification, at rendering things intelligible. Sometimes it fails—as with rendering the mind-brain nexus intelligible, or the nature of free action, or a prioriknowledge. Sometimes it delivers respectable results: the analysis of definite descriptions, modal logic, and the nature of the good (though all three areas are not without controversy). The science of philosophy makes progress in matters of clarification; it increases the intelligibility of things. But even when it doesn’t succeed that is its ideal—it is intelligibility-oriented. Language is one domain in which the project of clarification can be applied; our conceptual scheme is another; and the world in general is a third area of potential clarification. Total clarity is the aim of every philosopher (or should be).
One particularly sharp way in which questions of intelligibility come up is in the shape of the logical paradoxes. These are peculiar to philosophy and vividly illustrate its essential character: philosophy generates them and then it tries to solve them. Philosophy is a paradox-obsessed subject. There are many such: Zeno’s paradoxes of motion, the Sorities paradox concerning vagueness, Russell’s class paradox, the semantic paradoxes, and others. In addition to these we have assorted “puzzles”—kinks in our thinking that resist easy resolution. Many papers begin “The Puzzle of…” Both paradoxes and puzzles threaten intelligibility: they make seemingly straightforward things into confusing and confounding things. To resolve them some clarification is required, but this is not always forthcoming—they can be infuriatingly persistent (puzzlingly so). When paradoxes spread (as with the Sorities paradox) they threaten to undermine the intelligibility of everything. They are the nightmare of reason, and they are particularly disturbing to philosophers: for they threaten to undermine reason from within. What this shows from a meta-philosophical perspective is that philosophy is in the business of securing intelligibility, which is a none too easy thing to do. We don’t even understand how paradoxes arise: is it from our language or our thought or the objective world? And the last thing a philosopher wants is to discover paradox at the heart of his favorite theory (as with Frege’s set-theoretic reconstruction of arithmetic). Paradox is the ultimate philosophical embarrassment.
Philosophy is also a subject of extreme contrasts, and this too is part of its identity. The disagreements within philosophy are vast: idealism versus materialism, Platonism versus nominalism, consequentialism versus deontology, dualism versus monism, realism versus anti-realism, reductionism versus anti-reductionism. These are not just disagreements of detail but of fundamentals. There are even disagreements about whether whole swathes of reality really exist: do minds really exist, do bodies really exist, do moral values really exist? If philosophy is a science, it is a remarkably contentious one. But again, though this certainly sets philosophy apart from other subjects, it is just part of the very nature of philosophical questions: for these questions precisely concern the most fundamental issues about the nature of reality. If a subject sets out to deal with such fundamental questions, we should expect large disagreements to show up—that is just what philosophy is. It isn’t that philosophers as a group are particularly argumentative or stubborn or dim-witted; it is just that the questions inevitably produce these kinds of extreme opposition. That is what philosophy is about—it is the science of deep disagreement. It thrives on lack of consensus. Scientists are sometimes critical of the lack of consensus in philosophy compared to their own fields, but really there is nothing at all surprising here—philosophy is designed to produce deep differences of opinion. This is part of what makes it alive and exciting. It would be terrible—the end of philosophy—if a dull uniformity were to set in. In any case, consensus is not the hallmark of anything deserving the name “science”. What matters are rational methods, objective criteria of cogency, clarity of formulation, and standards of quality. 
Can philosophy ever come to an end? What would its end state look like? I think other subjects can, in principle, come to an end, and probably will before humans do. The sciences can end in one of two ways: all the problems are eventually solved, or some are not solved but never will be (at least by humans). There are only so many facts to discover, laws to state, and theories to be confirmed. But I think this is less clear for philosophical science: here it is not clear what the end state would look like. Can we imagine everyone deciding that materialism is true, say, and simply abandoning all other metaphysical theories as so much outmoded philosophical detritus? What could possibly lead to that result? It is not as if any new observations might be made that would settle the matter in favor of materialism. Or could it be settled once and for all whether consequentialism or deontology is the correct moral theory? Such debates seem internal to philosophy, part of what philosophy is. By contrast, disagreements in physics are hardly internal to it: they typically arise from lack of data or failure of theoretical imagination (or are really philosophical in nature). Neither of those diagnoses would seem to apply to philosophical disagreement. If anything could put an end to philosophy, it seems to be beyond our imagination—a literally inconceivable intellectual revolution. We don’t know what it would be for philosophy to end. Neither can we imagine the problems of philosophy being replaced by other problems hitherto unknown to the philosophical tradition: it couldn’t be that all our current philosophical problems are solved but news ones arise to take their place. What could these be? We have a pretty solid grasp of what the problems of philosophy are; it is hard to see how we could have missed a whole range of new problems. So our current problems are the ones that will stay in existence as the centuries pass by, probably never to receive definitive solution (short of a superhuman stroke of genius or a cerebral upgrade of some remarkable sort). Progress will no doubt be made on these problems, as it has been made in the past, but the idea of an end to philosophy seems impossible to fathom. Philosophy is really a very peculiar subject, quite unlike other subjects; the last thing we should do is to try to squeeze it into some other box. And its problems are what make it what it is, these problems having a unique character (“philosophical”). It may be rightly classified as a science (why not so classify it?), but that is not to say much about its inherent nature. Philosophy is about as puzzling as the problems it deals with. Meta-philosophy is as difficult as philosophy, because it is just another department of philosophy. 
 The OED gives two definitions of “science”: “the intellectual and practical activity encompassing the systematic study of the structure and behavior of the physical and natural world through observation and experiment”, and “a systematically organized body of knowledge on any subject”. Philosophy clearly qualifies under the second definition, but it arguably qualifies under the first definition too, once we allow for thought experiments and are not too restrictive about “observation”. For “observe” the dictionary gives “notice; perceive” and “detect in the course of a scientific study”: at a pinch we can make philosophical method fall under these definitions, since it may involve noticing certain things about concepts (or words) and it detects truths in its own way (sometimes called, misleadingly, “intuition”). Thus the philosopher may be said to “observe” (notice, perceive), for example, that knowledge is not just true belief. The operative terms in the dictionary definition are “systematic study” and “systematically organized”: rigor and system are the hallmarks of science. Academic philosophy qualifies; barroom chat does not.
 Discussions of the nature of philosophy are often tacitly normative: the author is recommending a particular approach to the subject rather than simply describing its actual content. I intend my remarks here to be descriptive: this is the nature of philosophy as it has actually been practiced—though I daresay many people will contest my conception of philosophy. I certainly don’t think it is an easy question to answer.