Zhang Zai introduced the concept of mental expansion. The present study examines three questions regarding this seminal concept. First, what is mental expansion? According to Zhang Zai, one can reach a state of partaking of all the things only if one is capable of expanding and maintaining one's morals and conduct. Furthermore, mental expansion involves surpassing the limitations of selfishness and prejudice. Second, how can one expand the mind in practice? For Zhuang Zai, it is possible to expand one's mind through practical procedures; it is a matter of letting one's conscience control and guide one's faculties of reason and sensibility. Finally, what is the connection between the concept of mental expansion concept and Zhang Zai's fundamental notion of Qi? In conclusion, implications of this concept of mental expansion for Zhang Zai's notion of the Way of Heaven and theory of Mind-and-nature will be discussed in this article.