=100.= Cleanthes took a special interest in the physical speculations of Heraclitus, on whose writings he composed four books[69], and in particular in the bearing of his speculations upon the nature of the deity. The belief in the dualism of God and matter, of the Word and the world, is attributed to Cleanthes as distinctly as to Zeno[70]; but on the other hand the conception of an overruling unity is much more pronounced in the later writer[71]. Hence from the first Cleanthes endeavours to give a wider meaning to the primary fire of Heraclitus, the creative fire of Zeno. For this fire he proposed the new term ‘flame’ (φλόξ)[72]; at other times he identified it with the sky[73], with the sun[74], and with the principle of heat[75]; and finally adopted the term ‘spirit’ (πνεῦμα, _spiritus_), which has ever since held its place in the discussion of natural theology. This term appears to have been at first intended to combine the conceptions of the creative fire and of the Logos[76], but it gradually came to have distinctive associations of its own. Like fire, ‘spirit’ is to the Stoics a substance, stuff, or body akin to the element of air, but associated with warmth and elasticity; it is conceived as immanent in the universe and penetrating it as the deity; immanent in the human body and penetrating it as the soul[77]. The elasticity of spirit is measured by its ‘tension’ (τόνος, _intentio_), by means of which its creative power pushes forward from the centre to the circumference: as for instance in the human body walking is effected by ‘spirit exercising tension towards the feet[78].’ The theory of ‘tension’ has an immediate application to ethics. When the soul has sufficient tension to perform its proper work, it operates according to the virtues of Wisdom, Justice, Courage, and Soberness; but when the tension is relaxed, the soul becomes disordered and is seized upon by the emotions[79]. [Sidenote: Theology of Cleanthes.]