Act utilitarianism refines many elements of Mills and Bentham’s theory (henceforth M+B theory) unlike M+B theory act utilitarianism focuses on the action that cannot benefit everyone the most in a given situation unlike M+B theory which focuses on the greatest good for either the individual or the group this overcomes the problem of an individual always acting in accordance to either the individual or group but allows him to act in either as long as it is as useful as possible. Another way it refines M+B theory is that M+B theory focuses on the foreseen consequences of an action rather than the actual consequences, focused on by act utilitarianism this allows individuals to analyse past acts by the individual or other individuals to allow to come to a decision on whether that will be the best action however M+B theories only focuses on the foreseen consequences of an action. Act utilitarianism adds flexibility to utilitarianism because it allows a moral agent to act in according to the thing which will benefit the most people in a given situation unlike M+B theory which only considers the bodily utility and pleasure of an action.
Act utilitarianism still doesn’t address many issues with M+B theory for example: It still is difficult to define happiness and also it doesn’t address how other things are of intrinsic worth such as love and freedom which makes it difficult to argue that act utilitarianism addresses major problems within M+B theory. Another problem that act utilitarianism doesn’t address is that even if you could give each possible pleasure a numerical value, the consequences of even the smallest of our choices on everyone are so vast that we couldn’t possibly calculate them all. Another problem that it fails to address is that it can be used still to justify almost any action for example; I could argue that it would be for the greatest good to kill of 49.9% of the population in a painless fashion for the benefit of the 50.1% of people. So act utilitarianism doesn’t stop utilitarianism from justifying almost anything. It doesn’t address the problem of the naturalistic fallacy for m+b theory: Just because people desire pleasure, this doesn’t make pleasure desirable. Put another way, just because the majority of people would prefer something, doesn’t meant that they ought to prefer it or that it’s right to do it. It doesn’t address that you can’t assign a value to an amount of pleasure. It is impossible to compare the pleasure of getting a new job with the joy of having sex or the satisfaction of washing your car.