One possible thesis for utilitarianism is that it is the only ethical theory that can adequately account for the well-being of all sentient beings. This is because utilitarianism does not make any assumptions about the intrinsic value of any particular person or group of people. Instead, it simply focuses on the consequences of an action and whether they will lead to greater or lesser happiness overall.
Another possible thesis for utilitarianism is that it is the most practicable ethical theory. This is because utilitarianism does not require us to make any difficult or impossible calculations about the relative value of different people's lives. Instead, it simply asks us to consider the overall consequences of our actions and to choose the option that will lead to the best outcome for the most people.
Finally, some argue that utilitarianism is the most just ethical theory. This is because utilitarianism does not discriminate against any particular person or group of people. Instead, it treats everyone equally and impartially. This means that the interests of the most vulnerable people will always be taken into account when making ethical decisions.
Of course, there are also some objections to utilitarianism. One common objection is that it can lead to the sacrifice of the rights of individuals in order to achieve the greatest good for the greatest number. Another objection is that it can be difficult to measure the consequences of an action and to know for sure whether it will lead to greater or lesser happiness overall.
Despite these objections, utilitarianism remains one of the most influential ethical theories in the world today. It is used by philosophers, policymakers, and activists to make a wide range of decisions, and it continues to inspire new thinking about the nature of ethics and morality.