Hey there,
Since the ancient greeks are so to say the "fathers (mothers) of the age of enlightenment (philosophers from this age relied on ancient greek philosophy), the way of thinking, logic, science.
I bought the book "Sofie's World" by Jostein Gaarder to get my first insights into the development of philosophy from ancient times to our modern, to get the key ideas to later be able to fully understand the original sources by the writers presented in this book.
Sofie is a curious girl, who finds letters from an unknown person in her mailbox. It turns out, that someone tries to give her lessons on philosophy, and she takes the challange. So she begins to discover the beginnigns of philosophy and how it has changed throughout the years. It is written like a dialogue between Sofie and her mysterious teacher.
So here is a little extract on Platos idea of the immortal soul:
An immortal soulWe have just got it, Plato saw the reality devided.
One part ist the world of senses- we just can get imperfect knowledge of this one, through using our five (imperfect) senses. Everything in the world of senses is flowing, nothing lasts too long. There is nothing really in this world of senses, there are things, that appear (in the sense of being build, or through evolution) and vanish.
The other world ist the world of ideas - of which we can get save knowledge through reasonably thinking.
The world of ideas is not to discover through our senses. To have a balance, the ideas (or shapes) are eternal and unchangable.
Following Platos ideas, human itself is a devided being. We have a body, that flows. He is bound to the world of senses, and shares the same destiny as everything in the world of senses (e.g. a bubble). All our senses are bound to the body, and are not too good to rely on.
But we also have an immortal soul - and he/she/it is the "home" of reason.
Because the soul does not consist of matter, there is the possibility for him/her/it to take a look into the world of ideas.
Now I said nearly everything. But there is much more, Sofie: Ich say: THERE IS MUCH MORE!
Plato thinks, that the soul existed before she entered our body: Once the soul was part of the world of ideas
But as soon as the soul arrives at the human body, she forgot about the perfect ideas. But what happens now, just when the human lives amongst the nature, and experiences it, the soul regains vague memories .
If he sees a horse, but a imperfect one. But this indicates the process of regaining memories of the perfect horse, what the soul once has seen in the world of ideas.
But this process also starts a longing for the real home of the soul (the world of ideas).
Plato called this longing "eros". The soul is longing for her origin.
From this very moment he/she/it experiences that everything is imperfect, wants to be freed from the body.
But this is just an ideal example, as not every humanbeing allows the soul to go on this journey back home.
Most people cling on the relfections of the ideas in the world of ideas, that are manifested in the world of senses. But they don't see the perfect horse (belonging to what the soul has seen when he/she/it was in the world of ideas before entering the human body), only the "bad" imaginations of the perfect horse in the world of senses. Plato describes the Way of the Philosopher. His philosophy could be read as a "how to be a philosopher".
When you see a shadow, Sofie, then you immediately start to think that the shadow has to come from something. Mabe it is a shadow of an animal. Maybe it is a horse. You think so, but you cannot be sure.
So you turn around to see what it really is, the real animal (not his/her/its) shadow, that is much more beautiful and is a clearer image as the shadow is.
BECAUSE OF THIS PLATO STATED ALL PHENOMENONS IN NATURE AS SHADOWS OF THEIR ETERNAL FORMS OR IDEAS (everything is just an image of what it was back in the world of ideas, when you look on it as being that lives in the world of senses).
But most of the people feel comfortable with their life amongst the shadows, they do not think about from which objects this shadows come from. They believe the shadows are all there is. And in fact, they forget about their immortal soul.
this would be the next step:
http://www.youtube.com/watch?v=Ux4mcnFC_-cLove,
pulsar