I am trying to devise an argument against Kantian ethics, but I would like some feedback before I get into it because I don't want to end up just attacking a strawman.
First, I utilize Bernard Williams' internal reasons thesis:
"The internal reasons thesis is a view about how to read sentences of the form “A has reason to φ”. We can read such sentences as implying that “A has some motive which will be served or furthered by his φing” (1981: 101), so that, if there is no such motive, it will not be true that “A has reason to φ”. This is the internal interpretation of such sentences. We can also read sentences of the form “A has reason to φ” as not implying this, but as saying that A has reason to φ even if none of his motives will be served or furthered by his φing. This is the external interpretation of such sentences, on which, according to Williams, all such sentences are false.
So first I establish that all actions are for an internal reason, and so to say that "A has a reason to φ regardless of his motives" is entirely meaningless (though you must separate internal reason from moral demand). So, Kant said that if we only act in egoism, we cannot have free will - to which I respond that yes, technically if we only act in accordance to our desires, we cannot will what we will, so we do not have MAXIMAL AUTONOMY. BUT if any part of us can make these robotic prescribed algorithmic choices like the Kantian free will can, we also do not have free will, for then an alien force is in some way steering us and taking the autonomy from our will to power. Now, yes I realize that Kantian ethics is not entirely alienated from desires, but still the Kantian free will is capable of transcending egoistic compulsion. We really only identify with our own internal values. When you value another person, it is still YOUR value, so if you can operate in accordance to duty when you have no internal valued reason to do so, I would say that your will has lost its autonomy to act in accordance to egoism.
NOW although I do suggest a psychological egoism (IN A SENSE), I go on to reformat the debate of psychological egoism vs. altruism via utilizing a no-self theory (basically the Buddhist concept of anatta) and I say that we DO sincerely act for the sake of others, but we still do not act in a prescribed duty. So first of all, my ontology is a processual holism (everything is process rather than substance, the whole is greater than its parts - the universe is an interdependent whole) and I also posit a panpsychism (experience and aim is a fundamental part of everything). SO by taking "the self" out of the conclusion, and seeing everything is greatly interdependent, I believe to have come to the conclusion that compassion is actually a metaphysical grasping of reality as one - the fading of the ego boundaries. ALTHOUGH we genuinely act for other people, there is no real essential self, and so everything is still fundamentally egoistic in the sense that everything acts in interest of "itself" and others, but simply only because we see others as a part of "ourself" - so the only real "self" we can refer to is THE UNIVERSE as a whole, because nothing in the universe can be meaningfully referenced in abstraction from the whole.
So this argument is kind of two arguments really, but still they are both important to each other; so I wanted some feedback to see if this is all just rubbish, but it makes a lot of sense to me. In conclusion, if we have any sense of autonomous nature at all, we cannot act in mere prescribed duty, for this would just be a robotic algorithm. Instead, we should see compassion as something egoistically beneficial, because if there are no real self boundaries, this should entail that being kind to others has a beneficial value.