Chapter 5 Animal Souls and Sensory Cognition (2024)

Among medieval philosophers, there was not much of a debate on whether dogs, cats, apes, or horses, and even more minute nonhuman animals, such as flies and bees, have souls. By and large, they all adopted the view of Aristotle according to which the soul is the ‘principle of life’. Hence, all living beings, from plants to humans, possess souls; otherwise they would not be alive. For ordinary people, the idea that nonhuman animals have souls seemed to be an odd idea. As Roger Bacon states in the Communia naturalium, the ‘crowd of laymen believes that only humans have souls which is why they laugh about the clergymen who say that dogs and other nonhuman animals have souls’.1 Similarly, Adelard of Bath, in his twelfth-century Quaestiones naturales, includes his nephew’s statement that the question of whether nonhuman animals have souls is ‘ambiguous to men of our time’. Adelard, however, who is his nephew’s interlocutor in this dialogue, stresses that only ordinary people would reply by clearly denying that animals have souls. Those, in contrast, who are familiar with natural philosophy do not at all hesitate to ascribe souls to other animals because they acknowledge that animals have ‘sensation’ (sensus), and sensation is a clear sign for the presence of a soul.2

Sensation or sensory cognition is indeed what characterises animal souls, and we shall see shortly what it means to have sensation. But before that it is important to note that the presence of sensation is only a sufficient, not a necessary condition for the ascription of a soul. The reason for this is that, despite the lack of any kind of cognition, plants have souls too, according to Aristotle’s widely-accepted theory: trees and flowers nourish themselves, they grow, and propagate, and so they have what was usually called a vegetative soul. Admittedly, this type of soul does not enable them to see their food or to feel their partners, for example. But it endows them with the basic functions of life: nutrition, growth, and reproduction.3 Since plants share these basic functions with nonhuman and human animals there is a certain continuity between them. This “principle of psychological continuity,” as it is sometimes called, is best expressed by the idea of the ‘great chain of being’ or the ‘ladder of nature’ (scala naturae).4

According to this picture, all (living) beings occupy the different steps of a ladder. And since the single steps build upon each other – it is much closer to a stairway than a ladder – there is continuity insofar as what is found at one step will also be found at the next step. For instance, nutrition, growth, and reproduction are found in plants, nonhuman, and human animals alike. The latter two, in turn, share not only the functions of the vegetative soul but they also have in common the functions of the so-called sensory soul. This is actually what sets them apart from plants and thus establishes a certain discontinuity, for plants count as living or ensouled beings, but unlike dogs or humans they are not animals.

In order to qualify as an animal, a living being needs to have sensation in addition to vegetative functions. In a nutshell, ‘animals’ (animalia) are living beings that have a sensory soul. In this sense, the concept ‘animal’ is a generic term which applies to various species of animal, both human and nonhuman.5 Thus, it establishes some sort of ‘generic community’ between various kinds of animals.6 It is certainly true that several authors tried to make a terminological distinction between human and nonhuman animals, for instance, by referring to the latter as ‘irrational animals’ (animalia irrationalia), ‘brutes’ (bruta), or ‘beasts’ (bestiae).7 Many of these expressions are pejorative rather than neutral and intend to emphasise the inferiority of nonhuman animals. Nonetheless, they do not exclude dogs, horses, or ants from the realm of animals. They only show that this realm can be further divided in one way or another. Since this was commonplace among medieval philosophers, to speak of ‘humans and other animals’ or ‘human and nonhuman animals’ is not to take an ideological stance towards the status of nonhuman animals. For medieval thinkers humans clearly are living beings that have a sensory soul, and this is what unites them with all other kinds of animals.8 But what does it actually mean to have a sensory soul? What is it that makes humans and other animals stand above plants on the scala naturae?

Above all, it is the possession of a number of organs by virtue of which they can engage in certain activities or operations or by virtue of which they have certain capacities. Of course, the possession of organs which have certain functions is nothing specific to sensory souls. The functions of the vegetative souls also depend on various organs: there are organs for ingestion, digestion, and for reproduction. What matters is rather that the organs of the sensory soul are other organs than those of the vegetative soul and so they can bring about other operations. As mentioned before, one can summarise the operations of sensory souls as cognitive operations. This means that they are “mechanisms by which animals acquire, process, store, and act on information from the environment,” to quote a definition of cognition that has become widely accepted in contemporary philosophy and psychology.9 By and large, this definition also catches the medieval notion of cognition.10 Still, it is important to note that medieval thinkers used to distinguish between sensory and intellectual cognition. One might therefore prefer to speak of ‘perception’ when talking about sensory operations. However, most of the authors covered by this study did not refrain from calling perception a kind of cognition (cognitio).11 Hence, it is neither anachronistic to call perception a kind of cognition nor is it wrong to claim that human and nonhuman animals have cognition because both of them clearly have various organs for sensory cognition.

Regarding the organs of sensory cognition a distinction was usually made between two different kinds of senses, namely, external senses and internal senses. The external senses are five: sight, hearing, smell, taste, and touch. All of them have corresponding organs: eyes, ears, nose, tongue, and skin or flesh.12Yet, if one compares the external senses of human and nonhuman animals, one will easily see that there are at least three important differences between the senses of different species. The first difference is that not all animals have the same number of external senses. While humans usually have all of these senses, there are other animals that lack certain of them. Bees, for instance, do not possess the sense of hearing, in the opinion of many late medieval thinkers, such as Thomas Aquinas. Even though bees are obviously frightened by loud noises and so seem to hear, this is only because they feel ‘the motion of the air’ (commotio aeris) that is produced by such noises, Aquinas explains.13 Lack of hearing is the reason why bees cannot be taught because ‘hearing is the sense of instruction’ (auditus est sensus disciplinae), as Aquinas remarks in the same place with reference to Aristotle’s De sensu et sensato. However, they are not seriously impaired by this lack because they still manage to find nectar and pollen and the combs they built are of an admirable geometrical regularity.14 Thus, one can be an animal without having all of the five senses. This is even more obvious with regard to those species that do have only one external sense. A prominent example is oysters, whose only external sense is touch. Therefore, they are only slightly superior to plants.15 Nonetheless, they qualify as animals because, in contrast to plants, they have at least one sense and hence can engage in sensory cognition.

Besides differences in number between the external senses of human and nonhuman animals there are also differences in physiology, that is, in the corporeal quality of the sense organs. The eyes of bees have a different structure than human eyes, for example, and similar differences occur in the physiology of any other of the four senses: the ears of elephants are much larger than human ears and so are the noses of horses. Although these differences might seem to be mere differences of shape or structure, they actually point to a third and more fundamental difference, namely, a difference in the way in which the various external senses function. In other words, there is a difference in the senses’ degree of perfection. Certain species, such as hawks, for instance, surpass humans with respect to sight as they are capable of spotting prey over large distances. The reason for this is that their eyes have a different ‘complexion’ (complexio), as Albertus Magnus explains in his De animalibus.16 And so from physiological differences result various functional differences. In many cases, nonhuman animals benefit from these differences in comparison to humans, insofar as they have more powerful external senses than we do. Recognition of this sensory superiority has been a commonplace among philosophers since Antiquity. In his De anima, Aristotle already mentions that our sense of smell is much worse than the sense of smell of many other animals, and in medieval encyclopedias dogs are often mentioned as an example of this superiority.17 Other examples are the sight of lynxes, the taste of monkeys, or the touch of spiders.18 In sum, there is a large variety among animal species with regard to their external senses.

However, perception via the external senses is not the only kind of sensory cognition. Beyond that, many animals can, for instance, also imagine or remember things they have sensed. But imagination and memory do not take place at the level of the external senses: if one imagines or remembers a rose, one does not presently see, smell, or touch a rose. Instead, one can, for instance, close one’s eyes and still have a mental image of a rose. Therefore, imagination and memory were considered to be so-called ‘inner senses’ (sensus interiores). The theory of internal senses was extremely influential in the later Middle Ages but its history is highly complicated as there was much disagreement about various aspects.19 There was, first, disagreement over how many inner senses there are at all and, in particular, over how many of these are shared by human and nonhuman animals. In most cases, four inner senses are mentioned: (i) common sense, (ii) imagination or phantasy, (iii) estimation, and (iv) memory. In addition, many authors also mention a cogitative power, though for some this was an additional power in humans, whereas others took this to be the human analogue of nonhuman animals’ estimative power. Similarly, some held that imagination and phantasy are one faculty, while others considered them to be different powers.20 Some authors also argued that there is actually only one inner sense which has different functions.21 Besides disagreement on the number of internal senses there was also much debate over where the single senses are located in the brain.22 And third, there was a wide range of views on how these senses function, especially in different species of animals.23

All of these differences are more than mere technicalities because the way in which one describes the inner senses determines one’s view of the cognitive capacities of animals. For the moment, it suffices to highlight three points that are particularly noteworthy in this regard. First, just as some animals lack certain external senses, so do some lack certain internal senses. Flies, shellfish, and worms, for instance, do not have memory according to Albertus Magnus.24 Flies regularly return after we have chased them off because they are incapable of remembering our hurtful blows. Dogs, in contrast, have an outstanding memory. A popular example among ancient and medieval authors is Argos, the dog of Ulysses, who after many years of separation was the only one to recognise his master after his return.25 Thus, there are not only differences in number but also differences in the degree of perfection of the inner senses and the memory of dogs is just one example. As regards imagination, this difference was an important criterion for differentiating between ‘perfect’ and ‘imperfect animals’ (animalia perfecta and imperfecta).26 While the former are capable of imagining a rose without presently perceiving a rose, the latter can only have a bundle of sensory impressions of a rose in their imagination when presently perceiving a rose. As in the case of the external senses, these differences were mostly explained by differences in the quality of the bodily organs, that is, the specific parts of the brain in which the inner senses reside. Consequently, whether one has a good memory or a bad imagination depends on the bodily mixture of certain parts of the brain, and much as there are more or less slight differences between the individuals of one species, there are more or less profound differences between the inner senses of one species and another.27

A third point that is particularly relevant for theories of animal rationality is the role of one specific inner sense, namely, the so-called ‘estimative power’ (vis/virtus aestimativa) or ‘estimation’ (aestimatio). This power is definitely one of the, if not the, most crucial inner sense of nonhuman animals because its function is to judge what is perceived. For instance, it is the power that determines whether the thing perceived is harmful or harmless. The estimative faculty somehow tells the organism what it should do in response to a certain perception. The classical example of how this power works is found in Avicenna, whom most scholars credit with inventing the theory of estimation.28 In his Liber de anima, Avicenna introduces the estimative power by referring to the example of a sheep seeing a wolf. The sheep, Avicenna says, not only perceives the sensory ‘forms’ (formae) of the wolf, such as the wolf’s colour and shape, its smell, or sound. In addition, it also perceives what, in the Latin, was rendered as ‘intention’ (intentio), namely, the wolf’s harmfulness.29 It is this intention of harmfulness which then triggers the sheep’s reaction of flight. And so the estimative power is essential to the sheep’s survival because without this faculty no reaction would be triggered.30

One might, of course, immediately ask whether the estimative power triggers nothing but instinctual reactions because the sheep’s reaction of flight is clearly an innate response that can be found in all sheep. But this question is a pivotal part of many of the discussions to be considered more closely in Parts 2 to 5. For now, it is enough to say that all animals, no matter whether they are human or nonhuman and no matter whether they are big or small, have sensory souls. This means that they possess a number of external and internal senses, even though these senses differ in number, physiology, functionality, and so forth. In every case, they provide cognitive access to the world and thus go beyond the basic functions of life provided by the vegetative soul. Sensory souls surely have much in common with vegetative souls. Most prominently, their operations are bound to bodily organs. For instance, the sense of sight does not work without eyes, and the faculty of memory is situated in a certain part of the brain where various sensory impressions can be stored, just as digestion requires an intestinal tract. This corporeal foundation of sensory cognition is the reason why sensory souls are often characterised as material souls: they are material in the sense that having a sensory soul means to have certain bodily organs. This corporeal nature of the senses is common to both human and nonhuman animals and so we share many things with dogs, horses, bees, and other animals.31