1. INTRODUCTION
The question of whether plants have life was of considerable interest to early scholars. Albertus Magnus, for example, devoted an entire section of De Vegetabilibus to the question of ‘whether plants are alive or not (an vivat planta vel non)’.Footnote 1 He wrote, ‘Now, since the first common principle appropriate to all plants and their parts is life [itself], which is found in plants, we shall first inquire about the life of plants […]’.Footnote 2 After reviewing the work of several philosophers, Albertus concludes that plants are indeed alive.
It was Aristotle who laid the foundations for the theory of the vegetative soul and the life of plants. The final form of his investigation is found in On the Soul, where he offers a clear definition of the relationship between life and soul: that which has a soul is alive, or as he puts it: ‘What has soul in it differs from what has not in that the former displays life’.Footnote 3 He begins On the Soul by declaring that the soul is the principle of life.Footnote 4 His criterion for something having life is that it has a soul. The life of plants can be inferred from the following: plants are alive because they have a vegetative soul. Aristotle demonstrates his view on the life of plants elsewhere. He writes: ‘Of natural bodies some have life in them, others not; by life we mean self-nutrition and growth and decay’.Footnote 5 The definition of life here presupposes the difference between plants and minerals, because Aristotle adds, ‘This power of self-nutrition … is the only psychic power they [plants] possess’.Footnote 6 Aristotle reinforces the existence of life in plants by describing another characteristic of the nature of plants: growth and decay. He writes: ‘Provided any one alone of these is found in a thing we say that thing is living – viz. thinking or perception or local movement and rest, or movement in the sense of nutrition, decay and growth. Hence we think of plants also as living, for they are observed to possess in themselves an originative power through which they increase or decrease in all spatial directions’.Footnote 7
In her study of Aristotle's notion of life,Footnote 8 Suzanne Mansion compared the definition of life in On the Soul, a work from Aristotle's mature period, with that in the Protrepticus, a work Aristotle wrote while he was still in Plato's Academy, to show the development in his definition of life. According to Mansion, Aristotle's earlier position was that life is defined by the existence of the sensitive and cognitive faculties. She calls this period ‘l’époque de la vision “perceptionniste” de la vie'.Footnote 9 However, this definition does not fully explain the elemental function of living beings, such as nutrition. The difficulty arising from this led Aristotle to abandon this definition in favour of the new definition articulated in On the Soul. In this work, Mansion argues, Aristotle adopts the approach of a biologist and characterizes life by the capacity for nutrition and growth.
Her analysis of the definition of life in each of these works is too meticulous to be treated in detail here, but what should be noted is the manner in which she reconstructs an evolution of Aristotle's thought regarding the notion of life. This is what I intend to do in respect to the work of Avicenna.
Initially, Avicenna was convinced of the existence of life in plants. In A Compendium on the Soul, which he wrote early in his career, Avicenna clearly held the view that plants are alive. In later years, however, his views changed. Surprisingly, in his treatise On Plants, the seventh book of the section on Natural Science in his work al-Šifāʾ, Avicenna declares, ‘It is impossible to attribute life to plants in any way (lā yaǧūzu an tajʿala li-al-nabāt ḥayāt bi-waǧh min al-wuǧūh)’.Footnote 10 He believes them to be lifeless, although he admits that they possess soul. It can thus be inferred that between A Compendium on the Soul, one of his earliest works, and On Plants, which he wrote while serving the Kākūyid ʿAlāʾ al-Dawla, he changed his mind. The goal of this paper is to explore how and why his thinking changed.
2. AVICENNA'S EARLY VIEW ON PLANTS
During the early stages of his career, Avicenna held the traditional Aristotelian view that plants have life. This stance can be seen in the principal work of his early career, A Compendium on the Soul,Footnote 11 written between 996 and 997.Footnote 12 In this work, he maintains that that which has a soul is alive. Avicenna states, ‘A living body (al-ǧism al-ḥayy) is a compound natural body (ǧism murakkab ṭabīʿī) which is distinguished from a non-living body by its soul and not by its body’.Footnote 13 At the end of the fifth chapter, a chapter on the animal soul, Avicenna reiterates his view that plants have life. He writes, ‘As for stars and plants, the sensitive faculty and the imaginative faculty are not given to them, even though each of them has a soul and life’.Footnote 14
Although Avicenna recognizes that plants are different from animals in terms of voluntary movement, the concept which later becomes the criterion for distinguishing the living from the lifeless, he does not seem to cast doubt on the idea that plants are alive at this stage. He writes:
We found that the compound ensouled bodies (al-aǧsām al-murakkaba al-mutanaffasa) – I mean bodies with souls (ḏawāt al-nufūs) – have two properties – motion (taḥrīk) and consciousness – in common.Footnote 15 Motion is common to all of them because they all make the motion of growth in quantity, but they differ in that some of them make local movement at will (bi-ḥasabi al-irāda) as well as the motion of growth, but others, such as plants, do not.Footnote 16
The realization that plants lack voluntary movement does not affect his conviction that plants are alive. He is consistent in his statements,Footnote 17 but this can be said only for this stage in his career. As we shall see, in the book of his al-Šifāʾ devoted to plants, Avicenna later argued that plants are not alive. We can regard the period between A Compendium on the Soul and On Plants as a transitional period during which he strengthened his belief that plants are without life. He refined his theory on vegetative and animal faculties, and inevitably arrived at the point where he could no longer attribute life to plants. However, he did not write much about plants or their souls in this period. All we can do, therefore, is to construct his theory on the vegetative soul from his writings on the theory of soul as a whole.
Let us look briefly at the works that were written in the transitional period. When Avicenna wrote al-Mabdaʾ wa-al-Maʿād, he still maintained that plants are living. The idea he formulated in A Compendium on the Soul was not easily discarded. He says that a body beginning to gain life first acquires ‘the life of nutrition, growth, and reproduction’.Footnote 18 Because these three functions are seen in plants, it is clear that at this stage, Avicenna does not doubt the existence of life in plants. In the State of the Human Soul, as Gutas suggests, Avicenna concentrates mainly on the rational soul; vegetative and animal souls are dealt with in detail only in his earlier Compendium on the Soul and later in al-Šifāʾ.Footnote 19 His view on the existence of life in plants during this period appears to have remained unchanged; we have to wait until al-Šifāʾ for a dramatic shift in his schema.
Avicenna does, however, give a clear statement on lifeless things in relation to the four elements, which do not have life. In Chapter 42 of al-Mabdaʾ wa-al-Maʿād, he explains that a compound body in equilibrium has life, but that the four elements cannot have life, as ‘the elements do not have life at all. If they are mixed and begin to be away from the conflict, then they begin to acquire life’.Footnote 20 He adds that a corrupted body does not have life because it does not have equilibrium, though it is composed of the four elements.Footnote 21
3. APPARENT REASON FOR LIFELESS PLANTS: PLANTS WITHOUT VOLUNTARY MOVEMENT
Avicenna's work On Plants marks a clear departure from the perspective he articulated earlier on the life of plants. Avicenna's criterion for something having life is higher than Aristotle's. In On Plants, he insists that that which has voluntary movement can be considered to be alive. He makes voluntary movement a condition for determining whether something is alive.
Before engaging in a closer examination of On Plants, it is worth noting some general observations about this work. This botanical section of al-Šifāʾ, consisting of seven chapters, was possibly written in 1027.Footnote 22 The philosophical argument appears in the first chapter. The second chapter deals with the generation and growth of plants, making comparisons with animals. The third chapter is about germination, and the fourth chapter addresses the growth and formation of parts of plants, and the role of moisture and heat in their formation. The fifth chapter deals with fruit and the function of leaves, while the sixth chapter deals with fruit, pericarps, seeds, thorns, and gum. Finally, the seventh chapter deals with the types and temperaments of plants, and their medicinal use.
As noted in the introduction to this paper, in the first chapter of On Plants, Avicenna expresses the surprising idea that plants are lifeless. What then are the conditions for a thing to be considered living? Avicenna improved on his earlier criterion with the following postulate: that which possesses the ability for voluntary movement is alive. The rationale he gives for this criterion is that living creatures must move voluntarily to seek food.Footnote 23 While animals move voluntarily to seek food, plants are fixed to one place and do not possess the will or ability to move. This movement is not limited to locomotion, as the example of shellfish shows.Footnote 24 Shellfish do not move from one place to another but are still counted amongst animals because they open and close their shells for nutrition by their own will. The clarity of this argument is such that there is no doubt that Avicenna had made up his mind to deny the condition of life in plants.
From the outset of his career, Avicenna had focussed on voluntary movement as a distinguishing feature between plants and animals. In A Compendium on the Soul, Avicenna had already stated that although both animals and plants have souls, there is a difference between them; the former are capable of voluntary movement.Footnote 25 Highlighting the significance of voluntary movement is not original to Avicenna; Aristotle also explains the difference between animals and plants in terms of the concept of voluntary movement. Although Aristotle does not make voluntary movement a condition for assessing life, he makes a clear distinction between animals and plants in terms of this characteristic; Aristotle writes:
At present we must consider local movement, asking what it is that originates forward movement in the animal. That it is not nutritive faculty is obvious; for this kind of movement is always for an end and is accompanied either by imagination or by appetite; for no animal moves except by compulsion unless it has an impulse towards or away from an object. Further, if it were the nutritive faculty, even plants would have been capable of originating such movement and would have possessed the organs necessary to carry it out.Footnote 26
Aristotle here explains that the nutritive faculty, the only faculty the vegetative soul has, does not cause voluntary movement. His argument of the limitation of the nutritive faculty might have given strong impact to Avicenna's view on the biology of plants. The realization of the insufficiency of the vegetative soul might have led him to think that plants are very different from animals, which obviously manifest life.
Avicenna was not the first scholar in the history of botany to doubt the existence of life in plants. Nicolaus of Damascus, though generally Aristotelian, could be seen as a forerunner of Avicenna in some respects. In his work De Plantis, Nicolaus expresses his doubt about the existence of life in plants. The work opens with the sentence ‘Life is found in animals and plants, but the life of animals is manifest and clear, whereas the life of plants is hidden and concealed’.Footnote 27 In Nicolaus' De Plantis, the condition for life is considered to be sensation; as he puts it: ‘sensation marks the distinction between life and death’.Footnote 28 He is suggesting that animals have life, while plants do not because they do not have sensation. Nicolaus also articulates that the principle of life in animals is sensation.Footnote 29 What then is the principle of life in plants? Nicolaus' argument on this point is not clear. He only hints at the issue in the following passage: ‘the Greeks call it the root and cause of life in plants, because the root supplies the plant with the cause of life’.Footnote 30 However, this does not show what Nicolaus believes to be the principle of life. He fails to clarify the nature of life in plants because he makes sensation the criterion for being alive, and sensation is not shared by plants.
The extent to which Avicenna was influenced by Nicolaus is uncertain, but we can assume that he had read Nicolaus' work. There are clear parallels between the two works. Avicenna, for example, writes in On Plants, ‘Some plants change into other kinds. For example, bergamot mint (nammām) changes into peppermint (naʿnaʿ), and basil changes into thyme’.Footnote 31 Accounts of the transformation of these plants can also be found in Nicolaus' De Plantis.Footnote 32 One may conjecture, therefore, that Nicolaus' De Plantis might have acted as a stimulus for Avicenna's reappraisal of his earlier view on plants.
In the works written after the botany section of al-Šifāʾ, Avicenna does not elaborate further on the concept of life in plants; he seems to be satisfied with simply describing the three faculties of the vegetative soul. Al-Naǧāt was written at about the same time as the two books on plants and animals of al-Šifāʾ.Footnote 33 In the first section of Book 2, Chapter 6, the discussion concerning plants is limited to the three faculties of the vegetative soul: nutrition, growth, and reproduction.Footnote 34 The physics section of the Dāneš-Nāma Footnote 35 has a chapter on the vegetative soul, which covers the emergence of the vegetative faculty in the balanced mixture of elements, and the three faculties of the vegetative soul, as well as their functions. He does not, however, touch on the question of the existence of life in plants. Dāneš-Nāma provides the source for al-Ġazālī's discussion on the vegetative soul in Maqāṣid al-Falāsifa. Al-Ġazālī discussed the distinction made by the philosophers of the three faculties of the vegetative soul, and their idea that the form of plants is nobler than that of minerals, originating in part from the idea that the growth found in plants does not exist in minerals.Footnote 36 Finally, in al-Išārāt wa-al-Tanbīhāt, Avicenna gives only a brief explanation of the three faculties of the vegetative soul, and no new perspective on the life of plants is offered.Footnote 37
It is the lack of voluntary movement that leads Avicenna to conclude that plants are devoid of life in On Plants.Footnote 38 This view of the nature of plants differs from Avicenna's earlier thinking, and I wish to propose that he had an unexpressed motivation that encouraged him to change his mind and deny the existence of life in plants.
4. UNDERLYING THEORY OF PLANT PHYSIOLOGY: PLANTS WITHOUT ANIMAL FACULTIES AND SPIRIT
Avicenna dares to claim that plants are lifeless, even though they have souls. As noted earlier, this argument stems from the view that living beings should be capable of voluntary movement. What I will suggest here is that this argument presupposes another theory on natural beings – one which emerges from Avicenna's medical works.
Avicenna's definition of life can be found in the Canon of Medicine.Footnote 39 The first part of this book was written in Ǧurǧān,Footnote 40 more than ten years before he wrote On Plants as part of al-Šifāʾ. The Canon of Medicine occasionally ranges beyond the medicine of the human body. For example, when reflecting on sensation in the human body, Avicenna asserts that plants, which do not have senses, lack something that beings with sensation possess: the animal faculty (qūwa ḥayawāniyya). Avicenna writes:
If the nutritive faculty prepared sensation and movement, then plants would be prepared for sensation and movement. Therefore, the preparer should be another thing which has a special temperament, referred to as the animal faculty.Footnote 41
What is important here is that the absence of the animal faculty does not only result in the absence of sensation and movement; it also leads to the absence of life because the animal faculty brings life to bodies. As Avicenna puts it, ‘They say that this faculty [the animal faculty] prepares life’.Footnote 42 He summarizes the functions of the animal faculty when he introduces it as a concept at the beginning of the section: ‘As for the animal faculty, they mean that it is the faculty that makes the organs prepare for accepting the faculty of sensation and movement, and the functions of life (afʿāl al-ḥayā)’.Footnote 43 This indicates that plants cannot have life because they do not possess the animal faculty. The faculty they have is only the nutritive faculty that allows them to have nutrition, growth and reproduction.
The animal faculty is, therefore, as Avicenna explains in the Canon of Medicine, concerned with three things: senses, movement and life. Beginning with the understanding that plants do not have senses and movements, Avicenna concludes that the only faculty possessed by plants is the nutritive faculty, and that the animal faculty, which is indispensable to life, is not given to plants. Later, in On Plants, Avicenna argues that plants are lifeless, though the reason he gives is outwardly different. Avicenna's view is, however, arguably underpinned by the understanding of the function of the animal faculty articulated in the Canon of Medicine.
A further perspective on the concept of life that is revelant to Avicenna's views is the relationship between life and heat. Avicenna's views on heat are articulated clearly in the Canon of Medicine: ‘The origin of life is the heart and the spirit, and they are very hot and tend to overheat. Life is maintained by the heat and growth is maintained by the moisture’.Footnote 44 With this in mind, the question arises as to whether Avicenna thought plants have heat.
Avicenna discussed the matter explicitly in On Plants:
Since cooking (ṭabḫ), which is the process of dissolving and separating by dissolution, does not happen without heat, heat is inevitable for the nourished body. Consequently, the life of the plants and, in general, the life of the being nourished is connected to moisture and heat.Footnote 45
This passage clarifies that he attributes heat to plants. It is not clear, however, how Avicenna thinks plants obtain their heat; he is silent on this question in On Plants and in other works. There is, thus, a question over the use of the phrase ‘the life of plants’ in this passage, but no explanation is offered. It can be argued that the word ‘life’ here means simply the period from generation to corruption. Note that in On Plants, the role of heat in plants is solely limited to coction; heat acts merely as an instrument for the nutritive faculty of the vegetative soul.
There is a further aspect to Avicenna's belief in the lifelessness of plants. Whilst life is related to the animal faculty and heat, both are related to the concept of spirit (rūḥ), as articulated in Part One of Canon of Medicine. Avicenna observes a close relationship of spirit to the animal faculty and to heat, saying that the animal faculty is ‘the faculty that is the first to appear in the spirit’,Footnote 46 and that ‘the hottest thing in the body is spirit, and the heart, which is the place of origination of spirit’.Footnote 47 In On Plants, however, the term ‘spirit’ does not appear at all throughout the book. The absence of this term is compatible with his denial of life in plants. When the animal faculty, which is related to life as noted above, is absent from plants, the spirit, a receptor of it, is not necessary. Heat exists in plants, but its role is limited only to a ‘cooking’ function. When heat is given only this simple role and is not connected to life, Avicenna might have felt hesitant to introduce the subtle and refined matter as spirit to explain heat in plants. Therefore, it is natural that plants do not have spirits as they are lifeless.Footnote 48
Reflecting on Avicenna's discussion of lifeless plants in On Plants, we can see that a different reason emerges for their lifelessness. The stated reason for their lifelessness is their lack of voluntary movement, but I suggest that the underlying and unexpressed reason Avicenna had in mind is their lack of the animal faculty. One explanation for this difference might be that he found it difficult to say that what has soul does not have a spirit – a carrier of faculties – although he was sure that plants are lifeless. He thus had to introduce another reason for the lifelessness of plants. As voluntary movement had marked a clear distinction between plants and animals from the early stages of Avicenna's career, he made it the criterion for life.
Thus, Avicenna did not conclude from the outset of his philosophical career that plants were lifeless, even though he acknowledged their lack of voluntary movement. The lack of voluntary movement alone could not lead him to claim that plants were lifeless; the evidence was too weak to draw such a conclusion. Only once he observed the absence of the animal faculty in plants was he able to be sure of the lifelessness of plants. Following this, heat was no longer considered the proof of life; its role was limited to coction.
5. CONCLUSION
Avicenna changed his mind on the matter of life in plants. His original opinion was that all bodies possessing soul, including plants, are alive. His opinion later changed, and he argued that plants are lifeless. As we have seen, Avicenna refers to the lack of voluntary movement as proof for this idea. Surprised by his claim, I sought to uncover the underlying motivation for his denial of life in plants, and concluded that Avicenna's research on the human body led him to recognize the function of the animal faculty, without which a body cannot be considered alive. That he is convinced of the lifelessness of plants may be seen in his use of the concepts of heat and spirit. Both concepts are deeply related to the concept of life, but in On Plants, heat is given only the role of coction, and the spirit is not mentioned at all.
Acknowledgement
I would like to thank Professor Charles Burnett for first bringing Ibn Sīnā's On Plants to my attention and for his comments on earlier drafts of this paper. The writing of this paper was made possible through grants from the Heiwa Nakajima Foundation, which enabled me to conduct research at the Warburg Institute in London.
