Spenta Mainyu — "the Holy Spirit" or "the Bountiful Spirit" — was the creative principle by which Ahura Mazda made the world. In the most rigorous Zoroastrian theology (the theology of the Gathas themselves), the cosmic dualism is technically a dualism not between Ahura Mazda and Angra Mainyu but between the two Spirits — Spenta Mainyu the Holy and Angra Mainyu the Hostile — both of whom had been present at the beginning of the world, and one of whom Ahura Mazda had chosen. Spenta Mainyu was thus the choice Ahura Mazda made; and through this Spirit, the Wise Lord acted in the world.

Yasna 30.3, the great passage of the choice, reads: "In the beginning there were two Spirits, twins by nature, of whom the Better and the Worse made decision in thought, in word, in deed. Between them, those who do good have rightly chosen, but not those who do evil." The two Spirits chose for themselves before any creature was made; Spenta Mainyu chose truth, life, and light; Angra Mainyu chose the lie, death, and darkness. Their choices set the polarity of the world. Each man, in his own life, repeats their choice in miniature.

In the later Sasanian and Pahlavi theology, the distinction between Ahura Mazda and Spenta Mainyu was sometimes collapsed: Ahura Mazda himself was identified with Spenta Mainyu, and the dualism was reformulated as between the supreme creator and the cosmic adversary directly. This was theologically simpler but philosophically less rigorous, and the Gathic original is often preferred by modern scholars as preserving the more refined original conception.

Spenta Mainyu was also the source of the seven Amesha Spentas — the Holy Immortals, who were aspects of his (and so of Ahura Mazda's) creative will. From him came Vohu Manah the Good Mind, Asha Vahishta the Best Truth, Khshathra Vairya the Ideal Dominion, Spenta Armaiti the Holy Devotion, Haurvatat the Wholeness, and Ameretat the Immortality, with himself as the seventh. Through the Amesha Spentas, the Holy Spirit's will became the ordered cosmos.