Gelber focuses on passages in GA where Aristotle treats heat and cold as agents which are said to be making (poiei), fabricating (dêmiourgei), solidifying (pêgnutai), putting together (sunistatai), or working (ergazetai) to accomplish something, and thereby playing a role in the formation of a living organism. Gelber s aim is to explain how heat and cold do this, and the significance, for Aristotle, of calling heat and cold the tools of soul in his explanation of animal reproduction.