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.
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.
Writing is something that you can never do as well as it can be done. It is a perpetual challenge and it is more difficult than anything else that I have ever done.
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.
I may have absolutely no control over what happens to us, but we can control how we respond. If we choose the right attitude, we can rise above whatever challenges we face.
So long as I'm the president, we will. I clearly see the challenge. I see the challenge of what these threats pose to our homeland, and I see the challenge of what these threats pose to the world.
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.
For we have always understood that when times change, so must we, that fidelity to our founding principles requires new responses to new challenges, that preserving our individual freedoms ultimately requires collective action.
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.
Our problem is that we often expect instantaneous solutions to such challenges, forgetting that frequently the heavenly virtue of patience is required.
That is honor's scorn
Which challenges itself as honor's born
And is not like the sire. Honors thrive
When rather from our acts we them derive
Than our foregoers.
We will build our defenses beyond challenge, lest weakness invite challenge. We will confront weapons of mass destruction, so that a new century is spared new horrors.