What is the role of freedom in the Stoic deterministic universe? How can we be happy and thus achieve 'salus' in a contingent, down-to-earth way that was so well developed before the spread of the Christian message? The analysis proposed in this paper focuses on the notion of human freedom in Seneca's writings, contextualised within his deterministic account of worldly reality and, more broadly, within Stoic cosmological conceptions. In the chain of fate, the human future may seem to be determined once and for all, and fundamentally inescapable. Nevertheless, the passage of time cannot bring anything unbearable to the wise individual, who is in permanent control of time, who approaches each day as if it were the last, and who neither longs for nor fears the future (De brev. vit. 2.2; 7.9). Freedom is thus exemplified by this specific way of life, which leads to moral improvement: 'freedom' means freedom from passions — especially those arising from projection into the future, such as irrational fear (NQ 7.1.5-6) — which makes it possible to rise above even fate itself (De brev. vit. 5M.3). For Seneca, then, human choice is balanced on the variable interplay between determinism, chance and freedom. Rational analysis serves to make sense of the world, to assess individual responsibility in the face of events, and to conceive of reality as an uninterrupted flow of causes that challenges our existential longings for happiness and freedom. Despite this ethical and theoretical rigour, Seneca's theological framework is one in which God is the supreme rational principle of reality, but also gives meaning and support to our existence, even in moments of hardship.