Cynic philosophy is often described as a philosophy of will. Its use of the word ἰσχύς would be an important step to the emergence of voluntas and to the Stoic tonos. However, determining whether Cynics defined ἰσχύς as a concept is difficult, as its occurrences are few. Instead, considering the essentiality of strength in virtue, whatever Greek words they use, may help us. One of the main questions for Ancients was: what does virtue require to exist and bring to happiness? For most of philosophers, sentences like these may be found in Diogenes Laertius’ doxographies. Strength is not considered as an overriding good. Plato classifies it as a bodily good and Stoics as an adiaphoron. Yet Antisthenes talks about a Socratic strength which is necessary to virtue. Only Xenophon’s texts can help us to understand what this expression means. My hypothesis in this work is that Cynics merely seriously take the matter of Socratic endurance as bodily strength and consider that knowledge is not sufficient for good actions. They do not spiritualize strength but consider it as the essential quality of virtue, which they get by a harsh training to pains. This is how voluntas can subsequently appear.