In which sentence does the author most clearly define his argument? adapted from The Goal of Sending a Man to the Moon
by John F. Kennedy

If we are to win the battle that is now going on around the world between freedom and tyranny, the dramatic achievements in space which occurred in recent weeks should have made clear to us all the impact of this adventure on the minds of men everywhere. Since early in my term, our efforts in space have been under review. With the advice of the Vice President, we have examined where we are strong and where we are not, where we may succeed and where we may not. Now it is time to take longer strides—time for a great new American enterprise—time for this nation to take a clearly leading role in space achievement. This in many ways may hold the key to our future on earth.
Recognizing the head start obtained by the Soviets with their large rocket engines, we nevertheless are required to make new efforts on our own.1(For while we cannot guarantee that we shall one day be first, we can guarantee that any failure to make this effort will make us last. We take an additional risk by making it in full view of the world, but this very risk enhances our stature when we are successful.2(We go into space because whatever mankind must undertake, free men must fully share.
I therefore ask the Congress, above and beyond the increases I have earlier requested for space activities, to provide the funds which are needed to meet the following national goals:
First, I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the moon and returning him safely to the earth. No single space project in this period will be more impressive to mankind, or more important for the long-range exploration of space. None will be so difficult or expensive to accomplish. In a very real sense, it will not be one man going to the moon—3(if we make this judgment affirmatively, it will be an entire nation. For all of us must work to put him there.
Secondly, an additional 23 million dollars, together with 7 million dollars already available, will accelerate development of the Rover nuclear rocket. 4(This gives promise of some day providing a means for even more exciting and ambitious exploration of space. This will perhaps go beyond the moon, perhaps to the very end of the solar system itself.
Third, an additional 50 million dollars will make the most of our present leadership, by accelerating the use of space satellites for world-wide communications.
Fourth, there will be an additional 75 million dollars—of which 53 million dollars is for the Weather Bureau. This will help give us at the earliest possible time a satellite system for world-wide weather observation.
Let it be clear that I am asking the Congress and the country to accept a firm commitment to a new course of action—a course which will last for many years and carry very heavy costs: 531 million dollars in fiscal '62—an estimated seven to nine billion dollars additional over the next five years. If we are to go only half way, or reduce our sights in the face of difficulty, in my judgment it would be better not to go at all.