Full Idea
The Stoic view is clear: the substance of soul comes about through some mixture of air and fire.
Gist of Idea
Stoics say the soul is a mixture of air and fire
Source
report of Stoic school (fragments/reports [c.200 BCE]) by Galen - The soul's dependence on the body Kiv.4.784
Book Reference
Galen: 'Selected Works of Galen', ed/tr. Singer,P.N. [OUP 1997], p.158
A Reaction
Most accounts seem to neglect the role of air (whatever that might be).