By ‘summum bonum’ (the highest good), Kant means that one should do what is moral, i.e. one’s duty because we know what is morally right or wrong instinctively. If we take our ethical nature seriously, we can see that it is what we should aim to achieve. Kant then goes on to argue that a good will, i.e. a person with the right moral intentions, will always seek to bring about the ‘summum bonum’, or the perfect state of affairs. In Kant’s own words, ‘the perfect accordance of the mind with the moral law is the supreme condition of the “summum bonum”.’ (Kant, Critique of Pure Reason) It is worth noting that at this point, Kant argues that ‘ought implies can’. Because we know that we ought to aim for the ‘summum bonum’ as it is our duty, it follows that it must be achievable. What we cannot achieve is not our duty. For example, if we see someone drowning, it is not our duty to do so if we cannot swim even though saving someone else is a moral action. In this situation our duty would be to find someone who can swim. The idea underpinning this belief is that human are autonomous. After all, as Kant himself argues, an action can only be said to be moral or immoral if it is done out of free will.
When Kant refers to ‘summum bonum’, he also refers to the idea that doing one’s duty should bring one fulfillment because it is the right thing to do. Happiness is the reward for being virtuous. In other words, happiness and virtue can be, and should be achieved together. Kant also argues that a ‘second element’ of ‘summum bonum’ exists as ‘happiness proportionate to morality’. Here we can see that Kant assumes that the world is fair. As aforementioned, however, Kant has made it clear that achieving happiness should not be the reason/ motivation for people to act morally. He believes that any ‘selfish interest’ will not help people reach moral perfection. Kant, however, also realizes that we as humans cannot ensure that happiness is added to virtue. On our own, we cannot ensure that we get what we deserve for our efforts even though we can strive towards virtue in our thought and conduct, because we are not omnipotent. In reality, good people are not always happy while bad people could be fairly happy in their own ways. Therefore, Kant argues that ‘summum bonum’ postulates the immorality of the soul (which outlives our body and will be judged after our physical death), as well as the existence of a moral, rational being, who can bring these two together and judge one according to one’s morality, perhaps in another life and another world. In order to do so, this being must be greater than any human being and also have the power to see through people’s selfishness when they act morally and award those who are truly God. This being we call God. In other words, God is the moral lawgiver of the universe. Therefore ‘summum bonum’ makes morality meaningful.
In conclusion, Kant believes that ‘summum bonum’ is the perfect state of affairs whereby virtue exists coincidentally with happiness. ‘Summum bonum’, however, cannot be achieve in this world simply because we are not omnipotent. Therefore ‘summum bonum’ postulates the existence of afterlife and God, who is omnipotent and reward those who act morally accordingly.