Quotations from Plato's Timaeus
"Time is the moving image of eternity."
"Time then has come into being along with the universe; and being generated together, together they may be dissolved, should a dissolution of them ever come to pass."
"If in a discussion of many matters … we are not able to give perfectly exact and self-consistent accounts, do not be surprised: rather we would be content if we provide accounts that are second to none in probability."
The central idea in Plato's Timaeus is his distinction between "what always is, and never becomes," and those transient beings that constantly become, and inevitably pass away.
Here is a summary of Plato's reasoning (from The Stanford Encyclopedia of Philosophy):
Some things always are, without ever becoming.
Some things become, without ever being.
If and only if a thing always is, then it is grasped by understanding, involving a rational account.
If and only if a thing becomes, then it is grasped by opinion, involving unreasoning sense perception.
The universe is a thing that has become.
The universe is visible, tangible and possesses a body.
If a thing is visible, tangible and possesses a body, then it is perceptible.
If a thing is perceptible, then it has become.
Anything that becomes is caused to become by something.
The universe has been caused to become by something.
The cause of the universe is a Craftsman, who fashioned the universe after a model.
The model of the universe is something that always is.
Either the model of the universe is something that always is or something that has become.
If the universe is beautiful and the Craftsman is good, then the model of the universe is something that always is.
If the universe is not beautiful or the Craftsman is not good, then the model of the universe is something that has become.
The universe is supremely beautiful.
The Craftsman is supremely good.
The universe is a work of craft, fashioned after an eternal model.