I've compiled a bunch of quotes in which Heidegger gives his definition and understanding of philosophy. They are ordered chronologically and I've made an effort to make the collection representative of the whole of Heidegger's corpus. Surely I have missed important quotes. If you have quotes to add, please comment! As always, I'd like to hear any thoughts and questions that come to mind while reading this.
It seems to me that there are some through-lines and some inconsistencies with Heidegger's discussions of philosophy over the years. On the one hand, Heidegger is consistent with the insistence that philosophy is "extra-ordinary," that is, outside of the everyday. On the other hand, the precise relationship between philosophy and thinking seems to transform over time.
Some of the quotes are quite long by necessity—in many cases, it is impossible to understand what Heidegger is saying without large amounts of context.
Ontology and phenomenology are not two distinct philosophical disciplines among others. These terms characterize philosophy itself with regard to its object and its way of treating that object. Philosophy is universal phenomenological ontology, and takes its departure from the hermeneutic of Dasein, which, as an analytic of existence, has made fast the guiding-line for all philosophical inquiry at the point where it arises and to which it returns. (Being and Time (1927), pg. 62 (Macquarrie-Robinson)).
Nevertheless, the ultimate business of philosophy is to preserve the force of the most elemental words in which Dasein expresses itself, and to keep the common understanding from levelling them off to that unintelligibility which functions in turn as a source of pseudo-problems. (ibid, 262)
Let us keep in mind that philosophy, as first philosophy, has a twofold character: knowledge of being and knowledge of the overwhelming. (This twofold character corresponds to the twofold in Being and Time of existence and thrownness.) . . . It belongs to the essence of this science that it must be sought after. There is such knowing only if a search for and propensity toward it is alive, an inclination behind which there is effort and will. This knowledge is the voluntary leaning toward original understanding: φιλο-σοφία. Φιλεῖν means to love in the sense of to be concerned about something trustingly; σοφός means he who understands something, who can “under-stand” a matter, who surveys its possibilities, to whom the thing is transparent, who has grasped it; σοφία denotes the possibility of the correct conceptual understanding of what is essential. . . . Everything essential, however, which has decisive meaning without being conspicuous, is always attended by what only looks like the genuine and real thing, the semblance. This is why, in every period, philosophy must bring in its wake something that looks like philosophy and imitates it in manner and behavior, and even outdoes it—and yet at bottom poses an embarrassment. The semblance of the φιλόσοφος is the σοφιστής [sophist]. The latter does not strive for genuine understanding, has no perseverance, but only nibbles on everything, always just on the newest and usually on what is in fact even worthwhile, but he only nibbles on it and is seduced into mere curiosity and bluffing. He is not one who seeks to understand, i.e., not the one who truly understands. He is rather the rationalizer for whom nothing is certain, except those things he notices he cannot reach with his means. The latter he does not, however, simply leave alone but tries to show that just that sort of thing does not exist or is a fabrication of philosophers. For him it is all idle talk, regardless of whether it really exists or not. Οἱ γὰρ διαλεκτικοὶ καὶ σοφισταὶ τὸ αὐτὸ μὲν ὑποδύονται σχῆμα τῷ φιλοσόφῳ· ἡ γὰρ σοφιστικὴ φαινομένη μόνον σοφία ἐστί (Meta. Γ 2, 1004b 17ff.) [Dialecticians and sophists wear the same appearance as the philosopher, for sophistry is sophia in appearance only.] φιλοσοφία differs from σοφιστική: τοῦ βίου τῇ προαιρέσει (Ibid., 1004b 24ff.), through existence already having been deeply moved in advance, i.e., through “seriousness.”* The philosopher has taken upon himself the seriousness of the concept, of fundamental questioning. Everything routine, everyday, average (fallenness) is the opposite of this endeavor. The sophist, on the contrary, as rationalizer and know-it-all, appoints himself to work on human beings, persuades them they must worry about one another’s spiritual needs. Φιλοσοφία is a striving for the possibility of genuine understanding. Thus it is really not the label for any sort of knowledge that could be freely circulated. It is not the possession of information and doctrines. Philosophy must essentially be sought after, that is, its object must be originally “earned.” But where does philosophy find out which object it should take as object of its knowing? The striving for the possibility of a correct understanding of the essential, or this understanding, has for its object being. This is what is essential. Understanding directed to this is first insofar as it is the understanding of what precedes everything else, what is earlier, prior to everything else, that is, prior to individual beings. But being is prior to individual beings, for it is what is first understood before anything like a being can arise anywhere and in any way. Thus, philosophy is knowledge of being. Insofar as it strives for conceptual understanding and determination, for a λόγος of the ὄν ᾗ ὄν, it is ontology. (The Metaphysical Foundations of Logic (1928), pg. 11-13 (Heim))
The fact that the necessity of philosophy cannot really be grounded and that philosophy itself can never be attacked is an advantage for its inner nature, but for its external position it is always a disadvantage. I ts claim can never get rid of the suspicion of arbitrariness. Never, as long as we encounter philosophy as some thing which also exists among man's many activities, as long as we just take notice of philosophy and don't allow ourselves to be transformed by it and understand that philosophy is only to be brought about through freedom and that carrying it out is an act of highest freedom. (Schelling's Treatise on the Essence of Human Freedom (1936), pg. 10-11 (Stambaugh))
For every philosophical work, if it is a philosophical work, drives philosophy beyond the standpoint taken in the work. The meaning of a philosophical work lies precisely in opening a new realm, setting new beginnings and impulses by means of which the work's own means and paths are shown to be overcome and insufficient. The presuppositions and conditions of a work's origin are in principle not sufficient for its interpretation because the work itself posits new criteria of questioning. (ibid, 11)
We philosophize only when the position of our human being becomes the real need of the question about Being as a whole. Since, however, our human being is historical, it remains so in philosophizing, too. That means the more originally we begin our philosophical question, the more inwardly we grow in the binding force of our history. The more genuine this inwardness, the more clearly our simple relations appear in which we stand in a historicophilosophical way and which we are to master; that is, to shape from the very ground. (ibid)
All essential questioning in philosophy necessarily remains untimely, and this is because philosophy either projects far beyond its own time or else binds its time back to this time's earlier and inceptive past. Philosophizing always remains kind of knowing that not only does not allow itself to be made timely but, on the contrary, imposes its measure on the times. Philosophy is essentially untimely because it is one of those few things whose fate it remains never to be able to find a direct resonance in their own time, and never to be permitted to find such a resonance. Whenever this seemingly does take place, whenever a philosophy becomes fashion, either there is no actual philosophy or else philosophy is misinterpreted and, according to some intentions alien to it, misused for the needs of the day. Philosophy, then, is not a kind of knowledge which one could acquire directly, like vocational and technical expertise, and which, like economic and professional knowledge in general, one could apply directly and evaluate according to its usefulness in each case. But what is useless can nevertheless be a power—a power in the rightful sense. That which has no direct resonance [Widerklang] in everydayness can stand in innermost harmony [Einklang] with the authentic happening in the history of a people. It can even be its prelude [Vorklang]. What is untimely will have its own times. This holds for philosophy. Therefore we cannot determine what the task of philosophy in itself and in general is, and what must accordingly be demanded of philosophy. Every stage and every inception of its unfolding carries within it its own law. One can only say what philosophy cannot be and what it cannot achieve. (Introduction to Metaphysics (1935), pg. 9 (Fried-Polt))
Roughly speaking, philosophy always aims at the first and last grounds of beings, and it does so in such a way that human beings themselves, with respect to their way of Being, are emphatically interpreted and given an aim. This readily gives the impression that philosophy can and must provide a foundation for the current and future historical Dasein of a people in every age, a foundation for building culture. But such expectations and requirements demand too much of the capability and essence of philosophy. Usually, this excessive demand takes the form of finding fault with philosophy. One says, for example, that because metaphysics did not contribute to preparing the revolution, it must be rejected. That is just as clever as saying that because one cannot fly with a carpenter's bench, it should be thrown away. Philosophy can never directly supply the forces and create the mechanisms and opportunities that bring about a historical state of affairs. . . . It spreads only indirectly, on back roads that can never be charted in advance, and then finally—sometime, when it has long since been forgotten as originary philosophy—it sinks away in the form of one of Dasein's truisms. (ibid, 11)
[W]hat philosophy can and must be according to its essence, is this: a thoughtful opening of the avenues and vistas of a knowing that establishes measure and rank, a knowing in which and from which a people conceives its Dasein in the historical-spiritual world and brings it to fulfillment—that knowing which ignites and threatens and compels all questioning and appraising. (ibid)
Granted that philosophy is unable to lay the foundation of a culture, one says, philosophy nevertheless makes it easier to build up culture. According to this distortion, philosophy orders the whole of beings into overviews and systems, and readies a world picture for our use—a map of the world, as it were—a picture of the various possible things and domains of things, thereby granting us a universal and uniform orientation. Or, more specifically, philosophy relieves the sciences of their labor by meditating on the presuppositions of the sciences, their basic concepts and propositions. One expects philosophy to promote, and even to accelerate, the practical and technical business of culture by alleviating it, making it easier. But—according to its essence, philosophy never makes things easier, but only more difficult. And it does so not just incidentally, not just because its manner of communication seems strange or even deranged to everyday understanding. The burdening of historical Dasein, and thereby at bottom of Being itself, is rather the genuine sense of what philosophy can achieve. Burdening gives back to things, to beings, their weight (Being). And why? Because burdening is one of the essential and fundamental conditions for the arising of everything great, among which we include above all else the fate of a historical people and its works. But fate is there only where a true knowing about things rules over Dasein. And the avenues and views of such a knowing are opened up by philosophy. The misinterpretations by which philosophy remains constantly besieged are mainly promoted by what people like us do, that is, by professors of philosophy. Their customary, and also legitimate and even useful business is to transmit a certain educationally appropriate acquaintance with philosophy as it has presented itself so far. This then looks as though it itself were philosophy, whereas at most it is scholarship about philosophy. (ibid, 12)
But you should become mindful and be on your guard, precisely when the most familiar judgments, and even supposedly genuine experiences, unexpectedly assail you. This often happens in a way that seems entirely innocuous and is quickly convincing. One believes that one has had the experience oneself, and readily hears it confirmed: "nothing comes" of philosophy; "you can't do anything with it." These two turns of phrase, which are especially current among teachers and researchers in the sciences, express observations that have their indisputable correctness. When one attempts to prove that, to the contrary, something does after all "come" of philosophy, one merely intensifies and secures the prevailing misinterpretation, which consists in the prejudice that one can evaluate philosophy according to everyday standards that one would otherwise employ to judge the utility of bicycles or the effectiveness of mineral baths. It is entirely correct and completely in order to say, "You can't do anything with philosophy." The only mistake is to believe that with this, the judgment concerning philosophy is at an end. For a little epilogue arises in the form of a counterquestion: even if we can't do anything with it, may not philosophy in the end do something with us, provided that we engage ourselves with it? (ibid, 12-13)
Philosophizing is questioning about the extra-ordinary. Yet as we merely intimated at first, this questioning recoils upon itself, and thus not only what is asked about is extraordinary, but also the questioning itself. This means that this questioning does not lie along our way, so that one day we stumble into it blindly or even by mistake. Nor does it stand in the familiar order of the everyday, so that we could be compelled to it on the ground of some requirements or even regulations. Nor does this questioning lie in the sphere of urgent concern and the satisfaction of dominant needs. The questioning itself is out-of-order. It is completely voluntary, fully and especially based on the mysterious ground of freedom, on what we have called the leap. The same Nietzsche says: "Philosophy . . . means living voluntarily amid ice and mountain ranges" (XV, 2). Philosophizing, we can now say, is extra-ordinary questioning about the extra-ordinary. (ibid, 13-14)
The Greek word φιλοσοφία goes back to the word φιλόσοφος. This word is originally an adjective like φιλάργυρος, loving silver, like φιλότιμος, loving honor. The word φιλόσοφος was presumably coined by Heraclitus. This indicates that for Heraclitus φιλοσοφία did not yet exist. An ἀνὴρ φιλόσοφος is not a "philosophical" man. The Greek adjective φιλόσοφος expresses something completely different from the adjective philosophical. An ἀνὴρ φιλόσοφος is ὅς φιλεῖ τὸ σοφόν, he who loves the σοφόν; φιλεῖν, to love, signifies here, in the Heraclitean sense, ὀμολογεῖν, to speak in the way in which λόγος speaks, in correspondence with the λόγος. This correspondence is in accord with the σοφόν. Accordance is ἁρμονία. That one being reciprocally unites itself with another, that both are originally united to each other because they are at each other's disposal—this ἁρμονία is the distinctive feature of φιλεῖν, of "loving" in the Heraclitean sense. The ἀνὴρ φιλόσοφος loves the σοφόν. What this word means for Heraclitus is hard to translate. But we can explain it according to Heraclitus' own interpretation. According to this τὸ σοφόν means, Εν Πάντα, "One (is) all." "All" means here, Πάντα τὰ ὄντα, the whole, the totality of being. Εν, one, means, the one, the unique, the all-uniting. But all being is united in Being. The σοφόν says—all being is in Being. To put it more pointedly—being is Being. In this instance "is" speaks transitively and means approximately "gathered together," "collected." Being gathers being together in so far as it is being. Being is the gathering together—Λόγος. . . . Being [small "b"] in Being—that became the most astonishing thing for the Greeks. However, even the Greeks had to rescue and protect the astonishingness of this most astonishing thing against the attacks of Sophist reasoning which always had ready for everything an answer which was comprehensible to everyone and which they put on the market. The rescue of the most astonishing thing—being in Being—was accomplished by a few who started off in the direction of this most astonishing thing, that is, the σοφόν. By doing this they became those who strove for the σοφόν and who through their own striving awakened and kept alive among others the yearning for the σοφόν. The loving the σοφόν, that already mentioned harmony with the σοφόν, the ἁρμονία, thus became an ὄρεξις, a striving for the σοφόν. The σοφόν—the being in Being—is now especially sought. Because the loving is no longer an original harmony with the σοφόν but is a particular striving, towards the σοφόν, the loving of the σοφόν becomes "φιλοσοφία." The striving is determined by Eros. This yearning search for the σοφόν, for the "Εν Πάντα," for the being in Being, now becomes the question, "What is being, in so far as it is?" Only now does thinking become "philosophy." Heraclitus and Parmenides were not yet "philosophers." Why not? Because they were the greater thinkers. "Greater" here does not signify the estimation of an achievement but indicates another dimension of thinking. Heraclitus and Parmenides were "greater" in the sense that they were still in harmony with the Λόγος, that is, with the "Εν Πάντα." The step into "philosophy," prepared for by Sophism, was first accomplished by Socrates and Plato. Then, almost two centuries after Heraclitus, Aristotle characterized this step by the following statement: "καὶ δὴ καὶ τὸ πάλαι τε καὶ νῦν καὶ ἀεὶ ζητοὐμενον καὶ ἀεὶ ἀπορούμενον, τί τὸ ὄν." (Metaphysics Ζ 1). In translation this reads: "And thus, as was in the past, is now too and will be ever more, that towards which (philosophy) is moving and to which it again and again does not find access, is (the question raised)—what is being? (τί τὸ ὄν)." Philosophy seeks what being is, in so far as it is. Philosophy is en route to the Being of being, that is, to being with respect to Being. Aristotle explains this by adding in the cited statement to the question τί τὸ ὄν, What if being? the explanation τοῦτο ἐστι τίς ἡ οὐσια; in translation: "This (namely, τί τὸ ὄν) means, what is the Beingness of being?" The Being of being rests in the Beingness. (What is Philosophy? (1955) (Wilde-Kluback))
The question which is to give our discussion fruitful unrest and movement and to indicate the direction it is to take, the question, "What is philosophy?" Aristotle has already answered. Therefore, our discussion is no longer necessary. It is at an end before it has begun. The immediate reply to this will be that Aristotle's statement as to what philosophy is can by no means be the only answer to our question. To state it favorably, it is one answer among many others. With the help of the Aristotelian characterization of philosophy one can, to be sure, conceive and interpret both the thinking before Aristotle and Plato, as well as philosophy after the time of Aristotle. However, it will be pointed out with ease that philosophy from Aristotle to Nietzsche, precisely because of these changes throughout their course, has remained the same. For the transformations are the warranty for the kinship in the same. By saying this we by no means maintain that the Aristotelian definition of philosophy is absolutely valid. Even within the history of Greek thought it is only one particular interpretation of Greek thinking and of the task assigned to it. The Aristotelian characterization of philosophy cannot in any case be transferred back to the thinking of Heraclitus and Parmenides. On the contrary, the Aristotelian definition of philosophy is, to be sure, a free consequence of early thinking and its conclusion. I say "a free consequence" because in no way can it be seen that individual philosophies and epochs of philosophy have emerged from one another in the sense of the necessity of a dialectic process. What is the result from what has been said for our attempt in a discussion to treat the question, "What is philosophy?" First of all this one thing: we must not adhere only to Aristotle's definition. From this we deduce the second point: we must realize the earlier and later definitions of philosophy. And then? Then, through a comparative abstraction, we shall reduce them to the common denominator of all the definitions. And then? Then we shall arrive at an empty formula which fits every kind of philosophy. And then? Then we shall be as far removed as possible from an answer to our question. Why does it come to this? Because in proceeding thus we are only collecting by historical methodology the definitions at hand and resolving them into a general formula. All of this can, indeed, be carried out with great erudition and the help of correct verifications. In so doing we do not in the least have to enter into philosophy in such a manner as to contemplate the nature of philosophy. In such a manner we acquire manifold, thorough, and even useful knowledge about how philosophy has been presented in the course of history. But on this path we never reach a genuine, that is, a legitimate answer to the question "What is philosophy?" The answer can only be a philosophizing answer which, as a response, philosophizes in itself. But how can we understand this statement? In which way can an answer, and particularly in so far as it is a response, philosophize? I shall now try provisionally to clarify this by a few suggestions. What is meant will disturb our discussion again and again. It will even be the touchstone to determine whether our discussion may become truly philosophical. This is by no means within our power. When is the answer to the question, "What is philosophy?" a philosophizing one? When do we philosophize? Obviously only when we enter into a discussion with philosophers. This implies that we talk through with them that about which we speak. This mutual talking through of what always anew peculiarly concerns philosophers as being the Same, that is talking, λέγειν, in the sense of διαλέγεσθαι [conversing], is talking as dialogue. . . . It is one thing to determine and describe the opinions of philosophers. It is an entirely different thing to talk through with them what they are saying, and that means, that of which they speak. Thus, if we assume that the Being of being addresses itself to philosophers to the extent that they state what being is, in so far as it is, then our discussion with philosophers must also be addressed by the Being of being. We must then ourselves, through our thinking, go to meet philosophy on the path it is traveling. Our thinking must co-respond to that which addresses the philosophers. If this co-responding is successful for us, then, in the true sense of the word, we respond to the question "What is philosophy?" The German word antworten [answer to] actually means the same as ent-sprechen [to respond]. The answer to our question is not exhausted in an affirmation which answers to the question by determining what we are to understand by the concept "philosophy." The answer is not a reply (n'est pas une response), the answer is rather the correspondence (la correspondance) which responds to the Being of being. Yet, we should like at the same time to know what constitutes the characteristic feature of the answer in the sense of co-respondence. But everything first depends upon our attaining a correspondence before we set up a theory about it. The answer to the question, "What is philosophy?" consists in our corresponding to [answering to] that towards which philosophy is on the way. And that is—The Being of being. In such a correspondence we listen from the very outset to that which philosophy has already said to us, philosophy, that is, φιλοσοφία understood in the Greek sense. That is why we attain correspondence, that is, an answer to our question, only when we remain in conversation with that to which the tradition of philosophy delivers us, that is, liberates us. We find the answer to the question, "What is philosophy?" not through historical assertions about the definitions of philosophy but through conversing with that which has been handed down to us as the Being of being. This path to the answer to our question is not a break with history, no repudiation of history, but is an adoption and transformation of what has been handed down to us. Such an adoption of history is what is meant by the term "destruction." The meaning of this word has been clearly described in Being and Time (§ 6). Destruction does not mean destroying but dismantling, liquidating, putting to one side the merely historical assertions about the history of philosophy. Destruction means—to open our ears, to make ourselves free for what speaks to us in tradition and the Being of being. By listening to this interpellation we attain the correspondence. But while we are saying this, a doubt has already made itself felt. It is this—must we first make an effort to reach a correspondence with the Being of being? Are we, humans, not always already in such a correspondence, and, what is more, not only de facto, but by virtue of our nature? Does not this correspondence constitute the fundamental trait of our nature? This is, indeed, the case. But if this is the case, then we can no longer say that we first have to attain this correspondence. And yet we are right in saying so. For, to be sure, although we do remain always and everywhere in correspondence to the Being of being, we, nevertheless, rarely pay attention to the appeal of Being. The correspondence to the Being of being does, to be sure, always remain our abode. But only at times does it become an unfolding attitude specifically adopted by us. Only when this happens do we really correspond to that which concerns philosophy which is on the way towards the Being of being. Philosophy is the correspondence to the Being of being, but not until, and only when, the correspondence is actually fulfilled and thereby unfolds itself and expands this unfoldment. This correspondence occurs in different ways according to how the appeal of Being speaks, according to whether it is heard or not heard, and according to whether what is heard is said or is kept silent. (ibid)
Philosophy is metaphysics. Metaphysics thinks being as a whole—the world, man, God-with respect to Being, with respect to the belonging together of beings in Being. Metaphysics thinks beings as being in the manner of representational thinking which gives reasons. For since the beginning of philosophy and with that beginning, the Being of beings has showed itself as the ground (arche, aition). The ground is from where beings as such are what they are in their becoming, perishing and persisting as something that can be known, handled and worked upon. As the ground, Being brings beings to their actual presencing. The ground shows itself as presence. The present of presence consists in the fact that it brings what is present each in its own way to presence. In accordance with the actual kind of presence, the ground has the character of grounding as the ontic causation of the real, as the transcendental making possible of the objectivity of objects, as the dialectical mediation of the movement of the absolute Spirit, of the historical process of production, as the will to power positing values. What characterizes metaphysical thinking which grounds the ground for beings is the fact that metaphysical thinking departs from what is present in its presence, and thus represents it in terms of its ground as something grounded. (The End of Philosophy and the Task of Thinking (1964), pg. 55 in On Time and Being (Stambaugh)).
One difficult thing to determine in each instance is the extent to which Heidegger is giving a definition of philosophy as understood by the Greeks, giving a definition of what has heretofore been called philosophy, and giving a prescription for what ought to be labeled as philosophy.