“Often we want to stop thinking, even for a moment. To forget all the difficulties and burdens of life, the sorrows and sufferings. It is much easier to just say, not to worry about anything. But this is the essence of our human consciousness, man's greatest enemy is himself.”

Actually Henry Wadsworth Longfellow, Driftwood (1857)
Misattributed

Address to United Nations General Assembly http://www.reagan.utexas.edu/archives/speeches/1987/092187b.htm (21 September 1987)
1980s, Second term of office (1985–1989)
Context: Cannot swords be turned to plowshares? Can we and all nations not live in peace? In our obsession with antagonisms of the moment, we often forget how much unites all the members of humanity. Perhaps we need some outside, universal threat to make us recognize this common bond. I occasionally think how quickly our differences worldwide would vanish if we were facing an alien threat from outside this world. And yet, I ask you, is not an alien force already among us? What could be more alien to the universal aspirations of our peoples than war and the threat of war?

Source: The Passage

Source: Fell

Source: Dictionary of Burning Words of Brilliant Writers (1895), P. 92.

Source: Thoughts in Solitude

Source: Feverborn