Here is a short precis I wrote for my meta-ethics class about Bernard Williams' argument for internalism about reasons. Internalism about reasons is, roughly, the Humean-ish idea that an agent has a reason to act a certain way only if it is somehow grounded in the desires of the agent. This is directly contrary to the natural law view, according to which what is reasonable to do or not to do is independent of the desires of the agent, and thus reasons are independent of an agent's desires:
In “Internal and External Reasons,” Bernard Williams presents an argument for thinking there is a problem with external reasons statements.
1. If something R can be a reason for action, then R can be a reason for someone A’s acting on a particular occasion O. [Premise]
2. If R can be a reason for A’s acting on a particular occasion O, then R can figure in an explanation of A’s acting in O. [Premise]
3. So, if R can be a reason for action, then R can figure in an explanation of A’s acting in O. [1,2 HS]
4. If R can figure in an explanation of A’s acting in O, then R cannot be an external reason. [Premise]
5. So, if R can be reason for action, then R cannot be an external reason. [3,4 HS]
6. If R cannot be an external reason, then R is an internal reason. [Premise]
7. So, if R can be a reason for action, then R is an internal reason. [5,6 HS]
Hence, the argument establishes that the only coherent notion of a reason for action is an internal reason for action, i.e., one which is relative to an agent’s subjective motivational set S. For now, let us grant premises 1 and 6. I’d like to think about premises 2 and 4 a bit. My point will be that, depending the sense of ‘explanation’ here, either one or the other premise will be implausible.
First, let us consider the interpretation of explanation where an explanation of an action is something that (directly) motivates someone to do that action. In this sense, maybe we should accept 4. For maybe it is only one’s desires, or at least something closely bound up with one’s desires, that could explain an action in the sense of being able to directly motivate it.
However, in this case, I think that premise 2 is implausible; for I don’t think that a reason for acting in an occasion has to be able to motivate you. Rather, a reason for performing P in circumstance C is some state of affairs or consideration that could make performing P in C practically intelligible. And something can do this without being able to motivate. Let me explain.
If you found me in a chair stabbing myself in the leg you would ask me why I’m doing what I’m doing. If I said to you, “I just desire to,” this wouldn’t really answer your question. There is still a clear sense in which what I’m doing is unintelligible, and wouldn’t make sense even if I obeyed some bizarre psychological laws which led me to “just desire” to stab myself in the leg. If however I said, “I am scared there is a tiny alien in my leg, and I need to get it out so it doesn’t kill me,” I will at least have described some state of affairs where, when my action is seen in light of this, my action does at least become intelligible (even if, in all likelihood, it is still utterly ridiculous and unreasonable). Such a state of affairs is a reason for acting a certain way.
But if this is a correct account of what reasons for action are, then 2 seems false. For it might be that a state of affairs S could make doing P intelligible, and thus counts as a reason for doing P, but my subjective motivational set makes me completely unable to even take S into consideration, and thus makes S unable to motivate me. This happens all the time. For instance, suppose I have no desire or motivation to quit smoking. However, the fact that smoking greatly increases my chances of dying certainly could make the action of quitting intelligible (i.e., if someone appealed to this fact when justifying their choosing to quit). So the fact that smoking greatly increases my chances of dying is a reason to quit. Yet, again, it might be that it couldn’t motivate me on any occasion.
All of this is to say that, on the interpretation of ‘explain’ where it is roughly equivalent to ‘motivate’, premise 2 seems implausible. On the other hand, if ‘explain’ means ‘make intelligible’, then premise 2 seems true, but premise 4 seems false, at least if reasons for action are, as I’ve explained, states of affairs which make acting certain ways practically intelligible.
 The “problem” is either that they are false, incoherent, or are misleading (and presumably should instead be rephrased as internal reasons statements). See the last paragraph of p. 297.
 This is my interpretation of Williams’ argument at the bottom left paragraph of p. 295. I may be misinterpreting Williams, but this seemed to me the best way to make his argument valid.
 I am assuming the sense of ‘can’ here means something along the lines of ‘is physically possible’.
 This account is largely adapted from G.E.M. Anscombe’s book ‘Intention’, sec. 37.