If the history of philosophy needs defending, the posts listed above ought to persuade anyone persuasible. Rather than add another log to the palisade, I thought it would be useful to try to understand why the question keeps coming up. (There’s ample evidence that it does come up every so often. You’ll find earlier stages of the discussion in Margaret Wilson’s Descartes (which was published, significantly enough, in a series called “Arguments of the Philosophers”), in Bernard Williams’ introduction to his Descartes (and the afterthoughts in A Sense of the past), in Alan Gabbey’s “Arguing with the ungrateful dead”—I can’t find a reference to it, but it circulated in samizdat in the late 1980s—, and Dan Garber’s introduction to Descartes embodied.)
The question keeps coming up, I think, because the history of philosophy has two faces. One looks toward intellectual history, itself a branch of cultural history, and the other toward contemporary philosophy. Call these the East Face and the West Face (that gives me room for two more if I need them…). The East Face thinks of philosophical texts in the manner of a historian: they are remains of the past which happen to be available to us in the Archive (see Foucault’s Archeology), and which we take as indices of past events, notably the acts of thought we take to be evidenced by the texts we study. The task of the intellectual historian (once the basic work of putting together the Archive is done) is to describe accurately and to explain those events. (Putting together the Archive is itself a very difficult and challenging task, I might add—see Thomas Tanselle’s work on editing and textual criticism; or take a look at André Robinet’s edition of Leibniz’s Principes de la nature et de la grâce.) That’s a rather atmospheric way of saying that we look at texts, try to determine the intentions behind the speech acts of their authors, and—having thus understood the phenomena—try to understand why the authors did what they did. The process goes on at the level of phrases (e.g. Descartes’ semel in vita), arguments, whole texts, œuvres, schools. (I should add that in my view every responsible reader does history, at least occasionally. Only in those situations in which everyone understands everyone else so well that “hitches” in understanding—do you mean by ‘essence’ what I mean?—never get in the way will we not be engaged in tasks resembling those of the historian properly speaking. Consider the giants of the 50s—Quine, Carnap—: for today’s graduate student, I suspect, a great deal more needs to be motivated and explicated than when I first encountered them; in the 70s there were plenty of native Quineans and Carnapians from whom one could absorb the requisite understandings without having to crack a book.)
Nothing in that task involves knowledge of or reference to contemporary discussions of the “same” questions or concepts. Indeed it is often better to bracket what you know about the state of the art now so as to avoid anachronism. The historian of Renaissance perspective may find it pedagogically useful to refer to projective geometry (whose roots lie in the thinking of Desargues and others on the foundations of perspective drawing), but neither that nor, say, acquaintance with 20th-century “post-perspective” painting will assist the task of description and explanation—except obliquely, by contrast for example (perhaps a historian now understands the significance of perspective better now that it is no longer taken for granted; in this way the present serves the same purpose as any other contrasting period would serve).
The East Face of the history of philosophy faces away from the present with respect to objects, methods, and aims. The objects are of course the texts but also the intellectual constructs we build in order to understand them—the construct “Hegelianism”, for example, taken not as some sort of timelessly available option or position but as a historical movement. There may or may not be an intelligible supra- or extrahistorical notion of substance; for the historian this is a matter of indifference because such a notion is historically inert. The historian’s constructs (“substance in the seventeenth century”, the “Scholastic notion of substance”) are supposed to be, implicitly or explicitly, attributable to the agents whose acts the historian is concerned with. Some historians would speak here of “agents’ categories”, invoking a rule of thumb to the effect that it is historical nonsense to attribute the agents categories that for one reason or another they couldn’t have operated with (e.g. ascribing to Newton the notion of an affine space). Our grasp of those categories or more generally of alien ways of thought is got by immersing ourselves in the documents (this is part of what is meant by “contextualization”; no philosopher’s vocabulary is entirely his or hers alone, and so you learn that vocabulary not just from one canonical text but from a “population” to which your canonical text belongs). I should note that the historian’s understanding of philosophy generally may well reflect the “historicizing” of concepts, problems, etc. that he or she carries out when doing history; contemporary philosophers who think that philosophy in terms of a sort of decision tree of perennial problems and solutions will likely seem naïve or ill-informed; some friction may result…
Moreover, the East Face is resolutely antiteleological. Descartes’ philosophy was not “leading toward” Kant’s, nor Kant’s to Hegel’s; the thought of the past does not have our thought as its destiny. (There are minor exceptions to antiteleology, namely, when a philosopher or school announces a program which they then attempt to carry out: Husserl’s phenomenology was certainly directed toward making philosophy a rigorous science.) Knowing how things turn out is from this standpoint a distraction. (I realize that there are theories of interpretation according to which, for example, the meaning of a text is the sum of reponses to it. Interesting though such a theory may be as a sort of stipulation, I don’t think it is of much use to the historian.)
Antiteleology and avoidance of anachronism may lead the East-facing historian to studies that from the standpoint of the contemporary philosopher look quite useless. Avoidance of anachronism requires, among other things, that in your capacity as historical explainer you must ignore present-day estimates of importance: if Christian Wolff was the vehicle through which Leibniz’s thought was transmitted to Kant and his contemporaries, then the less than stellar Wolff with his endless numbered, cross-referenced textbooks is what you had better read; in the late 19th century the same goes for Herbert Spencer, who for a time had a lot more readers than Nietzsche. Similarly you may find yourself studying questions that, Mark Lance says, rest on lapsed presuppositions, or theories that are “not even wrong”—i.e. no longer seriously in contention, like the various versions of vital force, soul, entelechy that even into the early twentieth century had an important role in some philosophers’ understanding of life and organism. But tell your contemporary colleagues that you’re studying Fechner’s theory of plant souls and see what happens…
So much for the East Face. The West Face, as you might expect, does look toward the present. Historians of philosophy are, in relation to their nonhistorical colleagues, the “village explainers”—of the canon especially, but of all the past productions of philosophers if need be. We tell you what these “weird sayings” mean, from the genuinely cryptic Heraclitus to the apparently straightforward Mill. To do that we cannot just “translate” the text into one of today’s idioms; lack of fit in the denotation and connotation of terms may well preclude anything so simple; instead we have to teach you how to think like a Pre-Socratic, a Scholastic, a nineteenth-century Idealist—not globally, to be sure, but well enough so that you have a handle on their “style of reasoning” (to borrow Hacking’s term) as it applies to the issue at hand. This we do by whatever means will work: analogies, portmanteau terminologies, worked-through examples… In any case “splainin’” may be philosophy, but it isn’t history.
Clearly the village explainer’s task requires of the historian an attentive and open-minded attitude toward the present. If I want to explain Scotus to people steeped in Kripke and Lewis, I must start at least (though I don’t expect to end) where they are. Moreover, if I want my Kripke-Lewisites to want to have Scotus explained to them, I had better have a good sense of what they consider to be important, interesting problems. I may be hoping to jiggle their framework by offering something I regard as an alternative differing profoundly from the status quo (an alternative which is nevertheless worth our trouble to understand, and not just the East-facing historian’s). But it’s their framework, not Scotus’s, that determines how the conversation begins.
The people I consider the best historians know all this implicitly. Hacking, for example, is excellent not only when he faces East and establishes how it really was and why, but also when he turns around and persuades us that the more-or-less alien notions and reasonings he has excavated (“conceptual archeology”, as Catarina Dutilh Novaes calls it) can be made to illuminate issues that concern us now.
There is, then, a built-in tension in the history of philosophy as it is now constituted (i.e. with the historians lodged mostly in departments in which contemporary philosophy is the main event). When the historian turns East, he or she is likely to turn toward questions that the contemporary philosophers regard as matters of mere curiosity—at any rate, as of little concern to them. Watermarks matter to the historian because that’s one way you date documents, and an accurate chronology is one of the first requisites to an adequate description of events and so also to an adequate explanation (you can’t very well explain X’s position on substance in 1690 by reference to X’s reading of Y if X didn’t read Y until 1700). The nonhistorian, on the other hand, wants, at most, only the results of the historian’s labors: accurate dates, reliable texts, sound explanations. When the historian turns West (toward that setting sun the owl of Minerva rises with) this is the kind of thing, along with the initiation into alien ways of thought, that he or she will offer.
The contemporary philosopher, in this slightly idealized scheme, is rather like the “working scientist”, whose interest in any text, past or present, rests on the use to be made of it in their current project. That attitude is entirely justified with respect to getting on with the project. But I don’t think that your or my getting on with our projects ought to be the only measure of worth in philosophy.
I at least tend to feel “at home” only when I’m with similarly Janus-faced colleagues. Whether they face west more than east is not so important as that they acknowledge the value of both.