In this post, I will try to briefly outline this approach to ethics. This will be stated in point form
1. There are many entities that give reason to act. Value is one but there are possibly others. In general, let us call them maxims. (Note, we use the word more broadly than Kant did)
2. It is only rational to act on true maxims. False maxims do not provide any reason to act.
3. Ideal agents will act only on true maxims. They know what the true maxims are. They know what duties we have, what it means to be perfectly virtuous etc. This supposes that imperfect agents can act on false maxims.
For example, we can conceive of cases where imperfect agents like us desire things that to an extent that exceeds how much value they have to us. Or we desire certain things insufficiently. Addiction may be an example, where we desire something in excess to how much value it provides. e.g. desiring cigarettes in excess of the pleasure that they provide.
Note: This means that we may have to reject notions of value where somethings are valuable merely because we desire them, we always desire things which are valuable to the extent that they are so, and that value and desire have nothing to do with eachother.
4. An ideal agent is logically possible.
Some things have to be said about this agent. For starters, the agent has perfect judgement, and is not limited by others. i.e. if an agent sets his mind to a logically possible goal, there is no reason why the agent does not achieve that goal. i.e. he is nomologically omnipotent.
5. A society consisting only of ideal agents is logically possible.
This has to be argued a bit. For a start, it would seem odd if a society of perfectly virtuous people is not possible. From a perspective of reasons, if X is the ultimate reason to act, then X must be true for any and all agents. i.e. there should be no reason as to why any ideal agent cannot act from X. This is especially true when the agent is among other ideal agents. (As opposed to a situation where a virtuous agent is among malevolent agents who may be able to block the agent''s actions etc).
6. Maxims, which all ideal agents cannot act on simultaneously, cannot be what truly give reason to act. This follows from 5 as such a maxim which all ideal agents for some reason or another cannot act on cannot truly contain the ultimate reason from which to act.
7. The maxim must also be able to retain its meaning under universalisation.
For example, a pro thievery maxim would be incoherent as theft contains meaning only with respect to property rights. However, under universal thievery, there are no meaningful property rights. Therefore theft is unacceptable.
8. Given 2, 6 and 7, rational action by any agent is constrained by a filter. To qualify as rational, an action must be such that its driving maxim is universalisable.