The Summun Bonum is the highest or supreme good. This supreme good should be, according to Immanuel Kant, the aim of human beings.

The Summun Bonum provides for the highest human dignity and contentment.

We can attain this goal through the perfection of morality, which he defines as the unwavering adherence to the Categorical Imperative, as the fundamental moral rule:  

Act only according to that maxim whereby you can at the same time will that it should become a universal law. 

The Categorical Imperative is a complex idea. But it means basically to act always with reverence to universal goodness and the humanity of others. In other words, it is the demand by reason for moral perfection.

The Categorial Imperative is derived from what Kant calls Pure Practical Reason. This is the ability to determine the will independent of any desire. This contrasts with common practical reason that aims at the satisfaction of desires.  

Pure Practical Reason is revealed by the Moral Law. It arises from the nature of consciousness, or what is called a priori knowledge and reason.

We always employ pure practical reasoning in our moral decisions. For example, if you are deciding whether to deceive someone, you are aware of the truth of lying. Pure practical reason deals in truth and justice, or what is commonly called the conscience.

The moral law also reveals the freedom of consciousness, or free-will. That is, despite any bias or life experience, you always freely decide whether to lie or tell the truth. As Kant explains in his work, The Critique of Pure Reason, the moral law and free will reveal each other.   

Just as a good athlete will want to be the best, so a moral person will aspire to the Summum Bonum.

Kant felt that because people define their happiness so strongly with desires, the Summun Bonum would not be possible in a human lifetime.  

The practice of Buddhism is specifically aimed at the liberation from desire:  

This is peace, this is exquisite — the resolution of all fabrications, the relinquishment of all acquisitions, the ending of craving; dispassion; cessation; Nibbana. (AN 3.32)

Once you are liberated from desires, you naturally will the highest good. This is because there is no ego craving to satisfy. You do not need anything to be happy. You regain freedom of will, and your reasoning becomes pure.

With Enlightenment and Nibbana, the Summun Bonum becomes possible in a human lifetime.