Avicenna came to an unexpected conclusion in On Plants in al-Shifaʼ: 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. He holds this view in al-Manda’ wa-l-ma‘ad and the State of the Human Soul. Later, however, he discards the previous idea and concludes that plants cannot be said to be alive. The apparent reason he gives in On Plants is the lack of voluntary movement in plants. This rationale alone, however, is too weak to reject Aristotelian tenet. Searching for his real motivation, I assume that this conclusion comes through his investigation on the faculties of the soul, discussed mainly in the first part of the Canon of Medicine, which he wrote prior to the composition of On Plants of al-Shifa’. In the Canon of Medicine, he discusses the crucial role of the animal faculties in the maintenance of life. This insight might lead him to think that that which does not have the animal faculties are not thought of as being alive. Avicenna, therefore, had to conclude that plants, which do not have animal faculties, lack the qualifications to be regarded as 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.