In Stoic philosophy, pneuma (Greek: πνεῦμα) is the concept of the "breath of life," a mixture of the elements air (in motion) and fire (as warmth). Originating among Greek medical writers who locate human vitality in the breath, pneuma for the Stoics is the active, generative principle that organizes both the individual and the cosmos. In its highest form, the pneuma constitutes the human soul (psychê), which is a fragment of the pneuma that is the soul of God. As a force that structures matter, it exists even in inanimate objects.
In the Stoic universe, everything is constituted of matter and pneuma. There are three grades or kinds of pneuma, depending on their proportion of fire and air.
A fourth grade of pneuma may also be distinguished. This is the rational soul (logica psychê) of the mature human being, which grants the power of judgment.
In Stoic cosmology, everything that exists depends on two first principles which can be neither created nor destroyed: matter, which is passive and inert, and the logos, or divine reason, which is active and organizing. The 3rd-century BC Stoic Chrysippus regarded pneuma as the vehicle of logos in structuring matter, both in animals and in the physical world. Pneuma in its purest form can thus be difficult to distinguish from logos or the "constructive fire" (pur technikon) that drives the cyclical generation and destruction of the Stoic cosmos. When a cycle reaches its end in conflagration (ekpyrôsis), the cosmos becomes pure pneuma from which it regenerates itself.
The Stoics conceived of the cosmos as a whole and single entity, a living thing with a soul of its own, a spherical continuum of matter held together by the orderly power of Zeus through the causality of the pneuma that pervades it. This divine pneuma that is the soul of the cosmos supplies the pneuma in its varying grades for everything in the world.
Another Stoic concept which offered inspiration to the Church was that of 'divine Spirit'. Cleanthes, wishing to give more explicit meaning to Zeno's 'creative fire', had been the first to hit upon the term pneuma, or 'spirit', to describe it. Like fire, this intelligent 'spirit' was imagined as a tenuous substance akin to a current of air or breath, but essentially possessing the quality of warmth; it was immanent in the universe as God, and in man as the soul and life-giving principle. Clearly it is not a long step from this to the 'Holy Spirit' of Christian theology, the 'Lord and Giver of life', visibly manifested as tongues of fire at Pentecost and ever since associated – in the Christian as in the Stoic mind – with the ideas of vital fire and beneficient warmth.