Philosophy term defined by renowned
deontologist Immanuel Kant who argued that the
summum bonum was the highest possible state of good reachable when all human actions were inherently
moral.
Kant formed an a priori (without evidence) argument suggesting that since only God could reward moral and immoral behaviour his existance must be necessary to persuade humanity to attempt to achieve the summum bonum.
Somewhat critically, Brian Davies pointed out in his Introduction to Philosophy of Religion (1982) that Kant does not state why it is necessary for the summum bonum to be possible.
Kant's argument relates closely to the soul building argument of John Hick who believed that the world was a necessarily imperfect place - created with flaws to encourage humanity to overcome the natural evil present in the world. Hick believed that through experience of evil humanity would learn moral conduct and therefore achieve a utopian existance - which is of course what Kant believed the summum bonum to be.
NB: As with many latin words it can be spelled multiple ways with Summa Bona often being used, as well as Summa Bonna (which I am informed is gramatically incorrect). Of these spellings Summum Bonum is the most widely accepted.