Would it have been worth it, after all?
An answer to the question of whether philosophical practice is possible, however, is not the end of the story: it may still leaves open the question of whether it is desirable. Those who are optimists about the prospect for philosophical progress have a pretty clear answer: it is, they could argue, just part of what we mean by "progress," that you ought to pursue it if ou can get it. But the issue is somewhat trickier for pessimists. If philosophical progress is not possible, then there are two different lessons that could be, and have been, taken from that.
On the one hand, a pessimist might take the impossibility of scientific progress in philosophy to be a sign that philosophy is a dead-end job--thus, a pessimist about philosophical progress might also be a pessimist about philosophy. The argument goes something like this: scientific progress is what makes an intellectual effort worthwhile; but there is no hope for scientific progress in philosophy; therefore there is no hope for philosophy to be worthwhile. On this view, philosophy is regarded as a sort of sort of pseudoscience which aspires to scientific progress, but which (by its very nature) can never achieve it; and so it is best abandoned in favor of empirical scientific inquiry. Needless to say, this is not a view that most profesional philosophers are particularly fond of or comfortable with, but it does seem to have been the consensus of the Vienna Circle positivists towards more or less all traditional philosophical inquiry, although not necessarily to the use of philosophical method to get clear on the logical structure of empirical questions. It is, perhaps, much more popular with professional scientists (as some of the Vienna positivists were themselves), who are inclined to think of philosophy as airy speculation that retards the serious empirical work of science.
On the other hand, one might draw a radically different conclusion--that pessimism about philosophical progress allows for a sort of liberation from the expectations for scientific progress, and thus for a reasoned optimism about philosophy. Philosophers in this camp argue that when we recognize that it makes no sense for philosophy to make scientific-technical progress, we ought to also realize that it makes no sense for philosophy to aspire to it either. Rather than expect philosophy to prove its worth with scientific-technical progress, and then judge it worthless when it fails to, these philosophers argue that philosophical inquiry must be worthy in its own right. The perceived need for philosophy to prove itself in terms of some sort of scientific-technical progress is often diagnosed as a sort of creeping scientism, and repudiated as a drastic oversimplification of our intellectual life. This may have been the position of Ludwig Wittgenstein against the Vienna positivists--although if Wittgenstein saw any intrinsic value in philosophical inquiry he certainly didn't think that most people could profit from it. In any case, it was certainly the position of latter Wittgensteinians such as Peter Winch, as well as other contemporary philosophers such as Martin Heidegger. It is also argued (by its proponents, mostly) to represent a return to the conception of philosophy found in ancient philosophers such as Plato and Aristotle.
There is, finally, a sort of middle ground. The former view held that philosophy without progress has no value; the latter view held that philosophy without progress has value in itself (and therefore its value must come from some consideration other than scientific-technical progress). But this leaves open a third option: that philosophy has value, not in itself, but in its contribution to scientific-technical progress in other fields. Since this view values philosophy as a means to an end rather than as an end in itself, we might call it "instrumental optimism" about philosophy. Here the idea is that the pessimists are right to say that the value of an intellectual discipline can only come by achieving scientific-technical progress, that the (traditional) optimists are right to say that philosophy has value, and that both camps are right to say that philosophical progress cannot be achieved. But instrumental optimists would go on to argue that both camps go wrong when they assume that philosophical progress is the only sort of progress that philosophy could contribute to. On this view, philosophy is seen neither as a sort of pseudoscience, nor as something radically distinct from the sciences, but rather as a sort of incubator for new sciences. Instrumental optimists point to the development of the natural sciences, and then the social sciences, out of philosophy, and argue that speculative philosophy can have value as a place for new sciences and new research programmes within the sciences to be proposed, as well as a critical location for confusions that obstruct progress in the natural sciences to be exposed and cleared away. Philosophy, then, is seen as a sort of midwife (although certainly not in the Socratic sense): she does not give birth to any progress of her own, but proves her worth by making it possible for others to bring their progress into the world. Thus, whereas the value of mechanics or biology or psychology is taken to be internal to the practice (i.e., judged in terms of the progress of mechanical, biological, or psychological achievements), the value of philosophy is taken to be external (i.e., judged in terms of its effects on achievements in other fields, such as mechanics, biology, and psychology). This view is congenial to the conception of philosophy, most famously propounded by John Locke, as a sort of intellectual "underlabourer" to the sciences. It is also a view endorsed in various articles by Hilary Putnam, and may be the most popular viw amongst contemporary Analytic philosophers--especially those with a naturalistic bent.
Of course, just because a view offers some sort of dialectical synthesis doesn't mean that it is correct; the "middle ground" may turn out to be an unstable attempt to navigate between two ultimately irreconcilable positions. For example, there's hardly any reason to deny the truth of what the instrumental optimists say about the historical relationship between what some philosophers did and what natural scientists do today. But that doesn't necessarily mean that the philosophers were doing philosophy when they did work that contributed to natural science. Of course, they might have called what they were doing "philosophy." (Actually, it's far more likely that they called it "φιλοσοφια", since most ancient philosophers didn't speak English.) But if so, that just means that they meant something more expansive by the word than what we mean by it today, and we might think that there are perfectly good reasons for sticking to the narrower conception of philosophical method. Moreover, whatever the status of their works that eventually contributed to natural science, and whatever the value of those contributions it's very difficult to make a case that the philosophical value is best captured by its contribution to scientific posterity. (Is the lasting value of Aristotle's work better exhibited by his Physics or by his Metaphysics? By his reflections on horse's teeth or on the good life for rational beings?)
Lessons for Philosophy and Metaphilosophy
Philosophers are somewhat notorious for pursuing discussions to the point that even the most simple concepts take on the rarefied air of paradox, and the whole discussion collapses into a self-referential hall of mirrors. The fate of this article is no different; indeed, now it is even falling into self-referentiality about its own self-referentiality. But there is an important lesson contained in this about the nature of philosophical disputes: they exhibit their own peculiar sort of what Jean-Paul Sartre called "absolute inwardness:" every attempt to resolve a philosophical dispute essentially entangles us in disputes about the nature of philosophy itself. Meta-philosophical reflections on the nature of philosophy and the possibility of philosophical progress are not specialized side-notes to other philosophical work, but rather are involved in--and themselves involve--key debates in metaphysics, epistemology and philosophy of science, philosophy of language, and the theory of value. It may seem like philosophy is stuck with a terrible burden, if it is expected to explain itself as well as its subject-matter at every turn. But appearances may be deceiving: things may not be as weird as they seem. As Ludwig Wittgenstein was fond of pointing out, it may not be that remarkable that you use the same methods even when you philosophize about philosophy itself: "When you are learning spelling, 'SPELLING' is one of the words that you learn to spell. But you don't speak of 'spelling of the second-order.'"