Plato (470 - 399 BC), followed his mentor, Socrates, and became an Athenian philosopher. Plato, in turn, mentored Aristotle.
Plato believed that all human knowledge is simply recollection. He believed the soul was immaterial, i.e. immortal, and existed before inhabiting a body. In this form the soul had access to all knowledge. Therefore, when humans come to know something it is because our souls recollect what they knew before materializing in a body.
This is the same concept furthered by many of today's spiritual writers, including Neale Donald Walsch and his Conversations with God series.
Source: The Intellectual Devotional, David S. Kidder & Noah D. Oppenheim