In classical philosophy, it is distinguishable from psyche (ψυχή), which originally meant "breath of life", but is regularly translated as "spirit" or most often "soul".
Aristotle furthers this idea of being a "middle cause" by furnishing the metaphor of the movement of the elbow, as it relates to the immobility of the shoulder (703a13).
Respiration is the process by which breathing helps to cool and moderate the inner vital heat (thermotēta psychikēs) held in the heart.
In Stoic philosophy, pneuma is the concept of the "breath of life," a mixture of the elements air (in motion) and fire (as warmth).
[8] In the foreword to his 1964 translation of Marcus Aurelius' Meditations, Maxwell Staniforth writes: 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.
[14] 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.
[15] 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)[18] that drives the cyclical generation and destruction of the Stoic cosmos.
[19] In his Introduction to the 1964 book Meditations, the Anglican priest Maxwell Staniforth discussed the profound impact of Stoicism on Christianity.
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.
Philo explains that, in his view, pneuma is for the light breathing of human men while the stronger pnoē was used for the divine Spirit.
According to Diocles and Praxagoras, the psychic pneuma mediates between the heart – regarded as the seat of Mind in some physiological theories of ancient medicine – and the brain.
Around 300 BC, Praxagoras discovered the distinction between the arteries and the veins, although close studies of vascular anatomy had been ongoing since at least Diogenes of Apollonia.
In the corpse, arteries are empty; hence, in the light of these preconceptions they were declared to be vessels for conveying pneuma to the different parts of the body.
The pneuma, inhaled from the outside air, rushes through the arteries till it reaches the various centres, especially the brain and the heart, and there causes thought and organic movement.
[24] They thought that logic was indispensable to medicine, and Galen tells us that the Pneumatic school would rather have betrayed their country than renounce their opinions.
He recognised in the four elements the positive qualities (poiotes) of the animal body; but he often regarded them as real substances, and gave to the whole of them the name of Nature of Man.