TONOS is ‘tension’. This is a relevant principle of Stoic physics accounting for attraction/repulsion. Also it is a way of interpreting what gives rise to virtues and vices in the soul.
In order to better understand the notion of <tonos>, it is convenient to call to mind the notion of <pneuma>. This is the “breath of life”, a mixture of air (motion) and fire (warmth).
<Tonos> is ‘tension’ in our philosophy promoting the dignity of all the individuals. According to ancient Greek philosophers and physicians, there are three kinds of <pneuma>, depending on the proportion of fire and air. Firstly, there is the <pneuma> as state of tension (<tonos>). This <pneuma> gives stability and cohesion (hexis) to things. It is an energy existing even in stones and other inanimate objects. Nemesius, a 4th-century Christian philosopher, attributes the power of pneuma in Stoic doctrine to its “tensile motion” (<tonicê kinêsis>). In other words, the <pneuma> moves both outwards, producing quantity and quality, but, at the same time, it moves inwards, providing unity and substance. An individual is characterized by the equilibrium of its inner <pneuma>, which holds it together and also separates it from the world around it.
Secondly, there is the <pneuma> as life force. Thirdly, there is the <pneuma> as soul.
All in all, <pneuma> for the Stoics is the active, generative principle that organizes both the individual and the cosmos. In its highest form, it constitutes the human soul (<psyche>), which is itself a fragment of the <pneuma> intended as the soul of God.
As a force with the capacity of structuring matter, it exists even in lifeless entities. One way for actualizing the concept of <tonos> could be to interpret it as one of the effects of energy (either on a physical and on a spiritual level).