Aristotle thought of souls as forms travelling through time, both as patterns in space and a chemical process of self-maintenance. Thus they were efficient cause (instigator), formal cause (essence), and final cause (end) for living beings (for more on causes…). They repurposed matter to their own ends in a process called nutrition, but nutrition was not possible forever (think second law of thermodynamics or, if you prefer, recall Aristotle’s belief that all material things must decay). With a goal of eternal persistence, they reproduced, making new copies that could survive after they were gone. All souls behaved this way and all living things had this kind of soul – a vegetable soul.
Though all souls do these things, some souls had other properties as well, higher functions, if you will. Animals were those living things that responded to stimuli and made changes in the world around them. While I find Aristotle’s notion of vegetable souls reconcilable with modern science, I am less hopeful for the animal soul. It requires some different physics than we are used to.
Aristotle believed that forms could be acquired by souls as well as matter. This is somewhat problematic. When a soul acquires new matter (eats) that matter loses its old form and takes on a new one – the soul is both form and activity of adding matter to the form. When a soul acquires a new form, it does not give its matter to that form. Rather, it sequesters the form in a sense organ. Recall that form cannot exist without matter for Aristotle, so external forms are brought to the senses, where an image is “formed.” Debate persists on how this happens. The most literal commentators think that matter in the sense organ takes on the form (as in the liquid in the eye becoming green when it “sees” green). A more common-sense approach suggests that somehow souls receive forms in a symbolic or representative way. The form is taken in as a computer processor takes in file.
This sensation (or acquisition of forms) allows the souls of animals to interact with their environment. They not only input forms, they process them. They prefer some inputs to others and act to change the world according to their preferences. It’s not too difficult to see the parallel with nutrition. Forms are a necessary substrate for perceiving the world. Chase the prey. Flee the predator. Go around the rock. “Prey”, “predator”, and “rock” are all concepts, forms for Aristotle. The animal soul grows and perpetuates itself by acquiring the specific forms of the things around it. Just as we live in the vegetable sense by processing matter and energy from food, so we live in the animal sense by processing forms and relationships from our sensed environment.
Motion is intimately related to sensation, for our choosing means we have favored one form over another, selecting what to chase, what to flee, and what to avoid. For Aristotle, the animal soul is all about stimulus and response. He believed animals had the ability to change their environment in a way that vegetables and rocks could not.
When it comes to vegetable souls and powers of nutrition and reproduction, they map easily onto modern concepts in modern biology. All organisms repurpose matter to suit their ends and all living things make copies of themselves. This applies not only to vegetables, but protists, bacteria, archaebacteria, … arguably even viruses. When it comes to animal souls we have a harder time.
First, Aristotle’s animals do not correspond well to any modern categories. All organisms respond to stimuli. Plants are far more active than we once thought. Starting in the 1970s biologists began to recognize the complex biochemical senses and actions of plants. Some plants even move, like the Venus flytrap. Bacteria and other one-celled organisms can also move and respond to stimuli. So, if we were to apply Aristotle’s animal souls, we would have to apply them to just about every form of Earth life. Viruses would be debatable, though we debate about them anyway, so that’s less of a problem.
Second, we no longer have a clear concept of forms; it would be hard to say what the acquisition of forms means to a modern listener. Until very recently, most thinkers thought that thinking (and perceiving) required the things thought about to exist somewhere. Now we know that computers (and fungus) can solve problems purely algorithmically, following steps without ever conceiving of the problem or even the components. They respond automatically. Perhaps humans also solve problems automatically and our “concepts” are not involved. Instead they might just be our attempt to understand what we’ve done, after the fact. (For more on this problem of will and consciousness, check out this discussion I led a the University of Arizona.)
For Aristotle, there was a clear line between sensing moving things (animals) and all other living things (vegetables). The animal soul represents the active power and end of perceiving that enlivens animals, allowing them to sense, choose, and act in the world. It was not a second soul, added to the vegetable soul, but a single soul that encompassed both activities. It may not be useful to us today, but it gives great insight into how Aristotle thought and how philosophers approached questions of life through the ages.