He learned that from his Secretary of Treasury, whom he later made Supreme Court chief justice. Which, coincidentally, also provided the nation with some pretty good rulings for the later part of the 19th century. Ever since Lincoln, the nation has known the president can print money. We really didn't know that, before Lincoln. When Jackson killed the central bank, he sent the nation's money to state banks; a nice kickback for the states! We had no debt, we didn't need to print money. But today, we are in dire straits, much more dire straits than Lincoln was in when he fired up the printing press. Every leader knows it and every leader is too afraid to use it. So we try to pay a debt in the trillions with an income tax in the billions... we can barely pay the interest.