> Can you prove this?
Arguments can be constructed for or against such things, they cannot ever be proven. Very few things can be conclusively proven about the world. We could after all be just be brains in vats being fed a simulated reality and we'd never prove the fact. There is always that doubt.
The perhaps most damning argument is to consider a being without desire, fear, content, disgust, anger, and other such things.
What would compel this entity to any action?
Try to rob them with a gun and they will not fear you. "I'll shoot!", you'll say, and if they react at all, they'll say "ok".
Try to bribe them with delicious food, wealth, sex; and you'll get nothing. They do not desire it.
Commit an injustice in front of them and they'll likewise not be angered. Stop a crime and they'll find no reason to praise you.
Even if we permit them to feel pleasure or pain, which is generous and not necessary for the experiment, they will by our definitions not desire the pleasure and seek it out or fear the pain and try to avoid it. Permit them hunger and let them starve, and they will not desire it to end.
This entity has no state of the world that brings them joy since they do not know what it is, and no state of the world that brings them anger or resentment for similar reasons. The pleasure of the beautiful will not be sought out for they have no desire, and the nausea and revulsion of the hideous will not be avoided. As a consequence, the logical and consistent will not be preferred to the illogical and inconsistent.
Their ranking in a social structure will not matter to them, since they will not fear becoming an outcast, have no envy of the success of others, and so on.
Since that is the case, core to this being is that all states of the world are equal in their eyes. Could this entity using their perfectly intact faculty of reason derive a system of ethics? This seems unlikely, since normative statements do in conflict with this prefer certain states of the world to others.