Answer:
The Greeks
Step-by-step explanation:
In this statement following the assassination of Martin Luther King Jr., Kennedy uses the quote to propose a way in which the world can be kinder and more gentle. Before he presented the quote, he introduced it in the following way:
"Let us dedicate ourselves to what the Greeks wrote so many years ago: to tame the savageness of man and make gentle the life of this world."
The introduction that Kennedy provides of his quote shows that he obtained this quote from the Greeks.