Before the 17th century, it took extraordinary genius to make even small discoveries. Men such as Galileo Galilei, Isaac Newton and many of their contemporaries made great discoveries. But their greatest contribution was in developing and furthering the scientific method. This was an organized method of making new discoveries. By the end of the century science literally exploded with new discoveries. Ordinary educated scientists could now do what only geniuses could do before. Ancient Greeks depended on observation and philosophy to come up with theories about the physical laws of the universe. Beginning in the 17th century scientists now used scientific proof of their discoveries through experiments. They also defended their discoveries against their peers. This was an important part of the scientific method.