This one never fails to inspire and stir me.
"Tell me…do you not feel a spirit stirring within you that longs to know, to do, and to dare, to hold converse with the great world of thought, and hold before you some high and noble object to which the vigor of your mind and the strength of your arm may be given? Do you not have longings like these, which you breathe to no one, and which you feel must be heeded, or you will pass through life unsatisfied and regretful? I am sure you have them, and they will forever cling round your heart till you obey their mandate. They are the voices of that nature which God has given you, and which, when obeyed, will bless you and your fellow men." ~ James A. Garfield, in a letter to a friend