I know that I have never been so well pleased as when I could shift power from my own, on the shoulders of others; nor have I ever been able to conceive how any rational being could propose happiness to himself from the exercise of power over others.

Thomas Jefferson (1743–1826) American political philosopher, polymath, statesman, US President (1801–09)

Letter (1811-01-26) to Destutt de Tracy
    (Source)

Often just the second clause is quoted: "I have never been able to conceive how ..."