In general, I think, explanation of occurrence of some event (or state of affairs, or value of property, etc…) consist in specifying the reasons why it is such as it is. In this post I want to analyze few types of reasons, their interrelations and relations to few other issues, like physicalism, metaphysical vs nomological necessity and free will.
One of the possible reasons is causation in the Humean sense.
In the Humean sense, causation is a relation between two types of events, such that an event of type E1 is always followed by event of type E2. As far we are concerned with an explanation, knowing about this kind of causal relation, we can explain the occurrence of event of type E2 by appeal to the earlier occurrence of type of event E1 and appealing to the causal relation.
Physics, as far as I know, almost never appeals to Humean causation. Instead it provides us with equation which for given type of system, specifies the relations among different measurables of the system (usually given by equation of some kind). One of those measurables is time.
So, in this kind of explanation, when someone asks why is the state of the system such as it is (at time t2), one can appeal to the type of the system, appeal to the equation which specifies the behavior of such system, and appeal to values of the measurables at earlier time (or which is same, appeal to the set of values of measurables, which include value of t1, where t1 < t2)
Note 2.1: In physics, I think (someone correct me if this is confused way to look at it), those two things which are appealed to as reasons in the physical explanation, namely the type of the system, and the values of measurables, are not seen as fully divided, and sometimes one type of the system can change into another. For example pair of matter and anti-matter particles can be annihilated and change to photons.
Note 2.2: As far the empiricist wants to claim that the explanation might hold (even if we can’t ever be certain that it holds), she needs to say that the reasons she is specifying might be the actual reasons why the things are as they are. In this case, the empiricist needs to buy a form of idealism – namely the identity of reasons as thought and the actual reasons.
Note 2.3: The physicist reason can be appealed to as a reason for the cases of Humean causation. What we need is the way to abstract two (more or less vaguely isolated in time) occurrences in the system which would fall under the given Humean event types. And then show how for the system of the type from which those types of events can be abstracted, it is necessary that one is followed by another. Another thing that should be pointed here, is that as far somebody is searching for explanation, it is searching for some reason that is not known. However that doesn’t mean that none of the reasons are not known. So, to ask about cause is then to ask something like… what is this reason that I don’t know of, and by knowing which I would understand why the state of affairs is such and such.
Note 2.4: It is sometimes said that physics (or science in general) answers just how questions, and not why questions. But, I can’t see it. Science gives perfectly good explanations of why some things happens or not, or happens in such and such way or not. Of course science does leave some why questions around, like.. why is there physical laws in the world, why are they such as they are, and so on, but that doesn’t mean that it doesn’t answer any why questions.
Another type of reasons are a priori reasons. Those kind of reasons may, but don’t have to include appeal to time.
If someone asks “Why are just two of the Smiths at the party tonight?”, given that both the person who asks the question, and the person who is explaining it know that there are three persons in Smith family, an explanation can be “Joe Smith is sick”. The explanation implicitly appeals to the a priori knowable fact that if you have three things, and if only one thing doesn’t satisfy some criteria, then two things will satisfy the criteria. (this relates also to second part of (2.3))
Note 3.1: In fact, given that the explanation through physical equations involves math, we can say that appeal to physical reasons will probably include appeal to a priori reasons also. In order to explain how the given physical law and previous values of previous measurements necessitates the state of affairs now (again given through other measurables), usually some math knowledge will be required also.
Note 3.2: Sometimes the equations which model the behavior are relatively simple, but the systems show complex behavior which show signs of some order. I’m thinking here of chaotic systems. The appeal just to the laws (or equations) in this case also isn’t enough, but also a mathematical explanation of chaotic behavior is needed.
Note 3.3: The a priori reasons also are related to physical reasons in another way. We can distinguish two types of reasons when talking about physical reasons. We can talk about reasons why we think that given physical law holds – consequently determining reasons, and the reasons why the physical law actually holds (if it does, or alternatively to the amount that it does) – the antecedently determining reasons. The consequently determining reasons are reasons which are related to the need to explain some experiments, and depending on your preferred view on science, will be related to induction, falsification, abduction or something else. But on other side, take for example the reasoning behind the Special Theory of Relativity. There from the set of some assumptions (the invariance of physical laws in relation to translations/rotations in space/time and the constant speed of light), such laws as the laws of dilatation of space, and contraction of time are deduced a priori. And as far a physical reasons include a priori parts, we can say that those a priori parts are antecedently determining reasons, i.e. as reasons those are actual reasons why the physical law holds, and are added to the assumptions.
Note 3.4: Connected to last note, it is of open question, if the assumptions for which we have consequently determining reasons have further antecedently determining reasons. For example in the case of Special Theory of Relativity it is open question if there is a reason why the speed of light is constant, or why physical laws are invariant in relation to space and time. This possibility is related to the question if the nomological and metaphysical modalities coincide. If the physical laws have antecedent determining reasons, which are a priori, the physical laws will be metaphysically necessary (there wouldn’t be possible world with different set of physical laws).
Also there are teleological reasons. That is, people often do something in order to accomplish some goal. So, if someone asks “Why is the light turned on?”, what is probably required is the teleological reason. Someone turned the light in order to accomplish something. So the Humean causation answer “Because somebody turned on the switch.”, or the physics’ answer which would give complete description of the system and appeal to the state at earlier time, even if possible, won’t be satisfactory.
Note 4.1: Maybe it could be argued that one can find subject’s intentions in the physical description of that subject at some earlier time, so that somehow a translation is done from the state of the physical system into intention, so that this requirement for teleological explanation can be reduced to the physical explanation. I don’t believe this is possible because, I think, the physical description is merely one aspect of the world and it leaves aside (abstracts from) other aspects in such way that only thing that we can do is to find the correlation between the physical aspect and those other aspects. And it is only something that we can do, because we are aware of both aspects. The examples would be the correlation between the color of the object (color aspect that we can see), and the reflection characteristic of the surface of the object (aspect that we can figure out by certain type of measurements, and which is about function describing how much percentage from the photons of different frequencies are reflected). Also the correlation between the way we feel, and presence of some chemical in the brain, and so on. As I said, this kind of correlations are to be expected, as what we have are two aspects of one and the same thing, and the aspects are not self-subsistent. (Think of how when we raise a temperature of a thing, we also change its reflection characteristic. Both are aspects of the thing, and those two aspects will be correlated not because those two are separate things which are related by some law, but because both are merely aspects. Both are changing because the thing is changing.)
Note 4.2: Related to (3.4), even if the physical laws are metaphysically necessary, as (if) they are merely an aspect which correlates with other aspects, there is open possibility for the teleological aspect not to be metaphysically necessary. This is certainly an interesting option for exploring in relation to the issue of free will.