Who wrote the Emancipation Proclamation?

President Abraham Lincoln drafted the Emancipation Proclamation in July 1862. The Proclamation, which did not become law until January 1, 1863, freed all the slaves in the Confederate states. It did not, however, free the slaves living in slave states that had not left the Union, though that freedom would be granted in 1865.
In July 1862, Lincoln showed his entire Cabinet the Proclamation he had written and said he was convinced he must free the slaves. Some of his Cabinet members urged caution. Secretary of State William Seward advised Lincoln to be careful about the timing of the Proclamation, so that it would come from a position of strength, not weakness. He believed that Lincoln should unveil it only after a major Union military victory, so that it wouldn't look like a desperate attempt to win the war.