The Twelve Keys (Practica cum Duodecim Clavibus et Appendice, De Magno Lapide Antiquorum Sapientium) is an alchemical treatise attributed to Basil Valentine (Basilius Valentinus), a figure said to have been a Benedictine monk. The work was first published by Michael Maier himself in his Tripus Aureus (Frankfurt, 1618), a collection of three alchemical treatises that Maier edited and brought to press. The historical existence of Basil Valentine is uncertain: Maier believed he lived in Erfurt and belonged to the Benedictine order, but modern scholars debate whether the name masks a fifteenth- or sixteenth-century author, or is entirely pseudonymous. The Twelve Keys circulated in manuscript before Maier's edition, and each key was accompanied by an engraved illustration, twelve in total, plus one in the appendix.

The treatise presents the alchemical work as a sequence of twelve operations or "keys," each unlocking a stage in the preparation of the Philosopher's Stone. The keys progress from the initial purification of the matter through stages of dissolution, conjunction, putrefaction, and multiplication to the final projection. Each key is described both in allegorical prose and in an accompanying engraving, making the Twelve Keys an emblem-like work that combines image and text in ways that anticipate Maier's own practice in the Atalanta Fugiens. The treatise is noted for its vivid, often violent imagery: wolves devouring kings, kings bathing in blood, bodies dismembered and reconstituted.

Maier valued Basil Valentine's work so highly that he personally attended to the editing and publication of the manuscript. De Jong notes that Maier's Tripus Aureus presents the Twelve Keys alongside the writings of Thomas Norton and George Cremer, the three authors depicted together in the frontispiece of that volume, discussing alchemical problems beside a burning furnace (De Jong, pp. 18-20). The fact that Maier served as editor of Basil Valentine's text indicates not merely scholarly admiration but personal investment in disseminating this work.

Emblem XXIV of the Atalanta Fugiens draws directly on the first key of Basil Valentine. The motto reads: "The wolf devoured the king, and when burned, restored him to life." De Jong traces this precisely to the Practica cum Duodecim Clavibus, Key I, explaining that the king symbolizes the Philosopher's Stone, which must be destroyed by the wolf (antimony) and purified by fire, after which it rises immortal (De Jong, pp. 188-190). Maier includes an extended quotation from the first key in his discourse, describing how the grim, voracious wolf, subject to bellicose Mars but born of grey-haired Saturn, must devour the king's body so that fire can then purify the remains. The process must be repeated three times until the Lion has conquered the wolf.

De Jong also identifies references to Basil Valentine's eighth key in Emblem VIII, where the piercing of the philosophical egg by a fiery sword connects to Basil Valentine's declaration that Mercury is locked up in prison by Vulcan at the order of Mars (De Jong, p. 96). Maier further discusses Basil Valentine in his Symbola Aureae Mensae (p. 257 et seq.), where he treats him as a historical author of significant authority. De Jong's bibliography entry notes the uncertainty surrounding Basil Valentine's identity while documenting Maier's conviction that the works were genuine and important (De Jong, pp. 304-305).

The relationship between Maier and Basil Valentine is one of the most intimate in the Atalanta Fugiens. Maier did not merely cite the Twelve Keys as a distant authority; he brought the text to press, integrating it into his own publishing program. The wolf-and-king imagery of the first key became one of the most memorable emblems in the Atalanta Fugiens, its violence serving Maier's pedagogical purpose of making the abstract process of antimony purification vivid and unforgettable. The reader who sees the wolf tearing at the crowned king is encountering Basil Valentine's allegory as Maier chose to transmit it: through the serious game of emblem, music, and commentary.