Science is based on experiment, on a willingness to challenge old dogma, on an openness to see the universe as it really is. Accordingly, science sometimes requires courage - at the very least the courage to question the conventional wisdom.
We'll continue to speak out in a respectful way; never challenging somebody's love for America when you criticize their strategies or their point of view.
As a matter of fact, it's in our interests that we help reformers across the Middle East achieve their objectives. This is the fundamental challenge of the 21st century.
Today I challenge our nation to commit to producing 100 percent of our electricity from renewable energy and truly clean carbon-free sources within 10 years.
I know that I'm not the easiest person to live with. The challenge I put on myself is so great that the person I live with feels himself challenged. I bring a lot to bear, and I don't know how not to.
If the imagination is to transcend and transform experience it has to question, to challenge, to conceive of alternatives, perhaps to the very life you are living at the moment.