Avicenna came to an unexpected conclusion in the treatise On Plants in al-Šifāʾ: that plants are not alive. This judgment is surprising in view of Aristotle's opinion that that which has a soul is alive. This paper shows that there is a development in Avicenna's thought on plants' life. He begins with the Aristotelian view that plants are alive inasmuch as they possess a soul, as we see in his early A Compendium on the Soul. Later, however, through his investigation of the faculties of the soul, discussed mainly in the first part of the Canon of Medicine, he is led to conclude that plants could not be said to be alive. This paper indicates the importance of considering the influence of Avicenna's medical findings on his philosophical arguments when examining the development of his thought.