The ancient Egyptians thought of dreams as simply a different form of seeing, with trained dreamers serving as seers to help plan battles and make state decisions. The ancient Greeks and Romans believed that dreams were equal parts predictions of future events and visitations by the dead. Sigmund Freud considered dreaming an expression of repressed conflicts or desires, which were—no surprise, this being Freud—often sexual in nature. Here’s what modern day scientists believe your dreams actually mean.