The Tractatulus de Practica Lapidis Philosophici (Short Treatise on the Practice of the Philosopher's Stone) is a brief Latin alchemical text falsely attributed to Aristotle, preserved in the first volume of the Artis Auriferae, one of the great alchemical compilations that served as Maier's working library. Like many pseudo-Aristotelian alchemical writings, the Tractatulus exploits the immense authority of Aristotle's name to lend credibility to its alchemical teachings, though its actual composition dates to the medieval period and its content bears no relation to Aristotle's genuine philosophical works.

The Tractatulus describes the preparation of the philosopher's stone through four stages. In the first stage, the matter is decomposed. In the second, purification and cleansing take place. In the third, the matter is formed into a slightly solid, pliable substance by the addition of moisture to dry earth. In the fourth, this substance is hardened into stone, the lapis philosophorum. Each stage is described through allegorical imagery rather than direct procedural language, creating a narrative of transformation that Maier found irresistible as material for his emblem program.

De Jong identifies the Tractatulus as the direct motto source for Emblems IV and V of the Atalanta Fugiens. Emblem IV draws on the text's brother-sister marriage imagery, encoding the union of sulphur and mercury as the marriage of male and female principles. Emblem V, one of the most striking images in the entire sequence, draws its central conceit directly from the Tractatulus: a woman nurses a toad at her breast until she dies and the toad grows fat from her milk. De Jong quotes the source text's instruction to tie the nursing woman's hands behind her back so she cannot harm her son, place a toad at her breasts to feed, and wait until the woman dies in fire and the toad grows bigger from the milk.

De Jong's commentary explains the chemical process encoded in this startling image. The woman represents the element water, and the toad represents the element earth. In the third stage of the work, the purified, very dry earth must be made pliable by adding water to it. The drier the earth, the more water must be added, and eventually all the water is absorbed: the toad grows and the woman dies. The process illustrates the fundamental alchemical principle that coagulatio unius naturae non fit absque dissolutione alterius, the coagulation of one nature cannot occur without the dissolution of another.

The Tractatulus also provides Maier with the four-stage framework that structures the early emblems of the Atalanta Fugiens. Emblems IV through VIII can be read as a sequence corresponding to the Tractatulus's four stages, with decomposition, purification, moistening, and hardening depicted through successive allegorical scenes. This structural dependence on a single source text within a group of emblems is characteristic of Maier's method, which alternates between drawing on a single authority for clusters of related emblems and synthesizing material from multiple sources.

For the modern reader, the Tractatulus de Practica Lapidis offers one of the clearest examples of how a single alchemical source text generates the visual and narrative content of the Atalanta Fugiens. The bizarre image of a woman nursing a toad, which might appear to the uninitiated as pure surrealism, turns out to be a precise visualization of a specific chemical instruction from a specific textual source, identified by De Jong through careful philological detective work.