How can there be a metaphysics of ethics? What does this even mean? I am completely lost on what this means.
[For Kant,] How can there be a metaphysics of ethics?
Let's start by pointing out that Kant does believe there can be Metaphysics of Morals enough so that this appears in the title two of his works:
First, there's Groundwork of the Metaphysics of Morals (German: Grundlegung zur Metaphysik der Sitten) (1785).
Second, there's the The Metaphysics of Morals (German: Metaphysik der Sitten) (1797).
What does Kant mean by asserting there is a metaphysics of morals?
There's a really long answer that relates Kant to the entire history of philosophy, which we'll bypass here. The short answer is that Kant accepts the following claims:
- "Ethics" is not defined by our [or anyone's] pleasure / pain.
- "Ethics" is not defined by our [or any] society.
- "Ethics" is about good and bad
- "Ethics" can be understood systemically.
- The "metaphysics" is that he thinks we can work this out as a system and he's claiming it is real.
The metaphysical objects in question are
- The idea of freedom
- The idea of rationality
- The idea of a limited free rational will
- the idea of a causal world
There's many more features to be put on both the metaphysics and the ethics sides, but the simplest idea is that Kant is a realist about ethics (thinks that ethics picks out something real in the world much in the same way there are stars, flowers, winds, dogs, cats, ands squirrels).
Obviously not everyone today is a moral realist (i.e. believes there is a metaphysics of morals to be had) and not every moral realist agrees with Kant about the metaphysics (for instance, Aristotle is a type of moral realist but not a Kantian; Mill too; Hegel too; quite a few others as well).
Although Virmaior's answer is compelling and of course correct, I would like to offer a more specific alternative that tries to make Kant's choice of titles comprehensible based on his own texts. Kant himself justifies and explains his wording in the preface of his Groundwork of the Metaphysics of Morals [GMM] (1785), and I suggest reading it as a whole for a better understanding.
He basically makes a three-stage taxinomy of philosophy as a whole, as I will show quoting the corresponding bits. It is based on very classical distinctions, mainly coming from Aristotelian philosophy. Bear in mind that historically, all science, i.e. methods and contents that represent knowledge or make it possible, was considered philosophy. That is why the terms are used interchageably (within certain constraints, as Kant rejects certain metaphysical writings as philosophy proper).
Stage one: Purely formal vs. about objects
The first distinction is between purely formal "science" and something that has to do with some kind of particular material, therefore called material philosophy. His names for the purely formal philosophy is Logics, the science of inferences. Here, the 'content' is purely about the form of the relations between all possible objects of thinking, without including any particular content or material at all, therefore it is purely formal. As soon as these relations are dependent on the actual objects or material we are speaking about, we enter the realm of material philosophy:
All rational cognition is either material and considers some object, or formal and occupied merely with the form of the understanding and of reason itself, and with the universal rules of thinking as such, regardless of differences among its objects. Formal philosophy is called LOGIC, whereas material philosophy, which has to do with determinate objects and the laws to which they are subject, is once again twofold. (GMM, 4:387)
Stage two: Material philosophy as either of freedom or nature
So what does it mean that material philosophy is 'twofold'? For Kant, there are basically two realms of objects: Freedom and Nature. The corresponding branches of philosophy are Ethics and Physics. This means that for him, ethics is (or should be) a real science with scientific methods inquiring its objects just as physics does with objects in nature. And, of course, that there are real relations between real objects in the realm of freedom we can examine scientifically in the first place, i.e. he is a moral realist:
... For these laws [of material philosophy] are either laws of nature, or of freedom. The science of the first is called PHYSICS, that of the other is ETHICS; the former is also called doctrine of nature, the latter doctrine of morals. (ibid)
Please note: He here changes the terms speaking of the actual laws of freedom to a doctrine of morals and of nature.
Stage three: Empirical vs. a priori
This is the most tricky one to understand correctly. In very short: although a priori literally means something like 'before all experience', this is not meant to be understood in terms of time, but in terms of inference; Philosophy dealing with objects a priori deals with objects and relations necessary for any of the actual experience we have, i.e. of course we need to have experience first before we can even think about its necessary conditions.
Now, if there are two realms of experience, freedom and nature, each of them of course would inherit its own conditions a priori. Aristotle's book on the physics that looked beyond what is accessable directly through experience to its conditions was Metaphysics, so Kant simply applies this label to the same idea within ethics, making it two metaphysics distinguished by their objects:
All philosophy in so far as it is based on grounds of experience can be called empirical, that which presents its doctrines solely from a priori principles pure philosophy. The latter, if it is merely formal, is called logic; but if it is limited to determinate objects of the understanding it is called metaphysics.
In this way there arises the idea of a twofold metaphysics, a metaphysics of nature and a metaphysics of morals. Physics will thus have its empirical, but also a rational part; so too will ethics, though here the empirical part might in particular be called practical anthropology, the rational part actually moral science. (GMM, 4:388)
This leaves us with the following taxinomy:
That of course is not really complete and can as well as should be refined and extended to fit in all of his philosophy. In particular, I am convinced that his (critical) philosophy can better be understood as either philosophy a priori - that looks at conditions of the possibility (and laws) of judgements, distinguished by their objects (logics, as formal, being the exception) - or empirical or 'worldly' philosophy, as he will call it later - that simply describes experience. And even if so, his Metaphysics of Morals from 1797 contains a fair amount of empirical statements. But this is another story.