By 1127, Stephen of Pisa and Antioch translated the Arabic medical encyclopedia Kitāb al-Malakī under the title of Liber Regalis. This paper aims to analyze, in book IV, the chapters concerning virtus, actio and spiritus animalis based on a critical edition of both the Arabic original and the Latin translation. Thanks to an attentive analysis of the vocabulary chosen by Stephen and the discrepancies with respect to the Kitāb al-Malakī, the paper will show which doctrines about human thought and sensation the translator has conveyed to the Latin readers. Moreover, it will be demonstrated that the definition of sanitas discussed at the end of book IV plays an essential role inside the encyclopedia, being the keystone of the whole medical science. Finally, the paper will argue that the knowledge of this very book of the Kitāb al-Malakī has suggested to the translator the idea that medicine only plays an imperfect and preparatory role in the path towards sapientia.