Ensoulment

[MarcoP]

I was intrigued by Nauta's passage quoted by Koen and did some research. You are not allowed to view links. Register or Login to view. does not explicitly mention stars:

Tu causis animas paribus vitasque minores
Provehis et levibus sublimes curribus aptans
In caelum terramque seris, quas lege benigna
Ad te conversas reduci facis igne reverti.

You then bring forth, with the same bases, lesser living souls,
And giving them light chariots fitting their heavenly nature,
Broadcast them in the heavens and on earth, and by your bounteous law
Make them, turned towards you, with returning fire come back.

In another paper (Magis sit Platonicus quam Aristotelicus) Lodi Nauta provides more details:

[William of Conches says] that some people have condemned Boethius, believing that he had said that God created all souls simultaneously and placed each of them on top of a companion star, whence they proceeded into human bodies. But here Boethius, as William notes correctly, is following Plato, and so one should first find out what Plato thought, bearing in mind, as William reminds his reader on several occasions, that Plato often spoke about philosophy through a veil. In fact, Plato does not seem to have said anywhere that all souls were created together, but what he did say was that souls were placed by God on top of the stars (Timaeus 41d-e). 
William then offers two interpretations of this phrase ‘on top of the stars’:
(1) By the soul’s reason man transcends the stars and discovers the creator beyond them. Therefore, Plato said that God placed souls on top of stars (in the sense of above the stars), since man derives this power of transcendence from God. 
(2) Alternatively, God placed the souls on the stars in the sense that He made souls in such a way that the influence of the stars enabled them to exist in human bodies. For stars cause warmth, chills, infirmities and the like in men.
Having seen what Plato meant, William continues, we can see that Boethius spoke according to one of these two understandings.

So it is Plato (You are not allowed to view links. Register or Login to view.) who says that each soul has a star as its chariot:

And when He had compounded the whole He divided it into souls equal in number to the stars, and each several soul He assigned to one star, and setting them each as it were in a chariot He showed them the nature of the Universe, and declared unto them the laws of destiny

"Timaeus" was one of the few works by Plato known in medieval times (through a Latin translation by Cicero).