Article 2, section 1 of the Constitution states, "No person except a natural born citizen, or a citizen of the United States at the time of the adoption of this Constitution, shall be eligible to the office of president; neither shall any person be eligible who shall not attained to the age of thirty-five years, and been fourteen years a resident within the United States."
The pertinent part is underlined. The Founders wrote themselves a "grandfather clause" so that they would be eligible to run for President since their parents were foreign born.
Right. Now where does it define 'natural born citizen'?