Plato's Allegory of the Cave
In Plato's Allegory of the Cave, people are chained facing a wall. A fire is across the cave, casting shadows which are all the people can see. They would think the shadows are all that exist. Suppose someone escaped these chains. They would see real objects for the first time, and see the fire casting the shadows. If they left the cave, they would see even more things, objects and creatures which would be incomprehensible to those in the cave who had only seen shadow objects. Of course, this would mean that to those who remain in their chains the person who escaped would be misunderstood and seen as a madman
Using our five senses, we can only see the "shadow" world. To see the true nature of things people must look beyond their senses and break free of mental chains. Philosophy breaks these chains, revealing the true nature of the world to the mind of the philosopher. However, especially in the time of Plato, philosophy was seen as dangerous and philosophers such as Socrates (Plato and his brother Glaucon's mentor) were put to death.