There are two main means whereby new philosophical subdisciplines are generated today.
The first takes a science distinct from philosophy as a subject of study in its own right. This is the cause of the generation of ‘philosophy of’ disciplines like philosophy of physics, biology, mathematics, etc.
The second operates via the assumption that there is something into which being and some other matter can be divided as members of a common genus, from which the other member of this genus can be separated out as a distinct object of study. Correlatively, the disciplines studying these other matters are themselves divided against metaphysics as the science of being.
The first mode suffices to generate disciplines that, while they retain some interest for a time, maintain a subordinate place within the overall organization of the philosophical sciences.
Furthermore, the level of activity generated in this kind of subdiscipline will be proportionate to the admiration held for it in the wider culture, i.e. proportionate to the prominence that science holds in the culture. For instance, for as long as the relative esteem for the sciences remain as they are at present, the quantitative output in philosophy of physics will surpass that in philosophy of biology.
Lastly, the existence of such disciplines is distinctly modern, in that it puts in action a presupposed distinction between philosophical and scientific method: such disciplines can arise today only on the presupposition that the methods practiced by these sciences are themselves insufficient to capture their own activity.  This is why today there can be a philosophy of psychology, but no philosophy of philosophy-of-mind.
The second mode, much older, is that whereby new core disciplines in philosophy are generated. Take the phrase “Being and “, and fill in the blank, and that blank will itself correlate to a distinct core philosophical discipline at a given period. In Ancient Greek philosophy, being is divided against becoming; accordingly, metaphysics is divided against Aristotelian physics, a contrast from which the former discipline receives its name. In the early modern period, being, as material being, is contrasted with both thought and thinking; accordingly, epistemology and philosophical psychology take up a central place. In Kant, we have the separation of being from the realm of the ought or value. This gives rise to modern ethics. In a move beginning with Leibniz and coming to fruition in Husserl, we see the pair being/appearing (which itself goes back to Plato) come to the fore; accordingly, phenomenology comes to form a core part of Continental philosophy, a place it still occupies today. And at the dawn of the analytic tradition, we have the pair being/language. This begets philosophy of language, and provides it with the centrality it had in both the regimenting tradition represented by Russell, Quine, and Tarski; and the ordinary language tradition of Ryle and Austin. This further explains why, in spite of its name, philosophy of language is a discipline of the second rather than the first type.
At the time that a new discipline of the second type arises, it is characterized in each case by a special relation to metaphysics. Often, the discipline becomes something of a rite of passage, a sine qua non for grasping being itself. This is the role taken up by physics in Aristotle, as well as by epistemology in the early modern period.
Other times, the object takes on the character of an insurmountable barrier. Thought does this in later modernity. Accordingly, the discipline studying that object may take on the character of a kind of guardianship, barring the path to superstition. Philosophy of language takes on this prohibitive role in the attitude of the Vienna circle.
Other times, the discipline may merely concede the study of being to another realm, typically on the assumption that the discipline can do well enough without it. This is the attitude of the early Husserl, who concedes the task of studying being to the positive sciences, taking phenomenology to open up a realm distinct from the reach of those sciences and interesting in its own right. This also remains the dominant attitude towards the ought in analytic ethics.
Other times, the discipline may itself revert into a kind of metaphysics. This happens when the object of that discipline itself begins to be perceived as all that there is to the world. This happens in two ways.
- The first way is when that object is identified with being itself. This occurs in a manner in the transcendental idealism of the later Husserl: since appearing belongs to everything within the sphere of consciousness, talk of being outside of that sphere is simply taken to be nonsense.
- The second way is when that object is taken not as being itself, but as source of being. Value takes on this role, for instance, in the ethics of Nietzsche: since beings are themselves meaningless, it falls upon the valuer to confer value on beings, and therefore accord them their being. Much earlier, form takes on this role in the philosophy of Aristotle, relative to the being of the material and elemental taken by some pre-Socratics to name being itself.
It is usually assumed, albeit often implicitly and unconsciously, that a wholly adequate philosophical understanding of everything there is would be one whose separate parts cohered in a maximally efficient way: within philosophy, this manifests itself in the desire to have one’s metaphysics ‘mesh’ with an adequate epistemology, semantics, etc. Outside of philosophy, it drives calls for philosophy to become interdisciplinary. It would be better for this new disciplinary egalitarianism to become conscious of the material root from which it arises: the presupposition that some beings simply don’t count as such.
 Even into the early modern period, for instance, the method the studying the physical world was sufficiently eidetic that study of this method could only amount to study of the method of philosophy itself; and so Galileo, for instance, could never be a philosopher of physics, but was instead a natural philosopher.
 This further gives the lie to the idea that philosophy should adopt the methods of the ‘exact’ sciences, since these sciences themselves witness that doing so would involve a loss in philosophy’s awareness of its own activity and guiding ideal.
 Accordingly, it explains its distinct character from Ancient ethics, for which the ought was not the subject, but the good.
 This is congruent with many ethicists denying this characterization. To the degree that the question of the relation to being does not enter into the discourse and activity of ethics, this silence itself governs the productivity of the field as a whole.
 This is why Nietzsche states, in the famous passage of Die fröhliche Wissenschaft, that we, who have killed god, must ourselves become gods to become worthy of the deed.
 This is the relation most clearly intended in Heidegger’s description of metaphysics itself as onto-theology, with Aristotle and Nietzsche as bookends to the story. This further is what undergirds Heidegger’s claim, in “Nietzsche’s word: God is Dead” that Nietzsche’s thought, in contrast with that of Kierkegaard, remains essentially within the purview of Aristotle’s.