 He defines the soul as “the principal agent inhabiting a body adapted to life (agens praecipuum, habitans in corpore apto ad vitam).” The soul is called an agent in the sense that it acts through instruments—e.g., heat, humors and spirits—by means of its own power. That it is the principal agent means that, even though its instruments act on the body, they do not operate by means of their own power but only through the power that they receive from the soul (III, 335–6). The organs governing our rational functions consist of fine and bright spirits exhaled from the pericardial blood. Although the heart is the source and origin of all the rational soul’s operations, the head is their workshop. In fact, the mind does not apprehend, nor is it affected, unless the spirits reach the brain (III, 365–6).