An Intuitive Argument Against Subjective Morality
P1) If a belief is made true simply by mentally affirming that it is true, then it is not worth arguing about the belief.
P2) If subjective morality is true, then all moral beliefs are made true simply by mentally affirming that they are true.
P3) It is worth arguing about moral beliefs.
C) Subjective morality is false.
Defense of premises:
In defense of P1, it could be said that the truth-maker for the belief is immediately intuited by the individual subject. Since each individual will immediately intuit their own mental affirmations, there is no value in debating with someone else what one’s own mental affirmations really are. One need only introspect to discover the truth-maker for the belief, which is merely the affirmation of the belief.
In defense of P2, that is just what subjective moral relativism holds. If you define subjective relativism in some other way, that simply is not my target.
In defense of P3 I point to empirical evidence that moral debates and disputes are held to be among the most worth-while discussions humans have.
The conclusion follows from the premises, i.e. QED.