Animals and plants as different kinds of engines
Keith S. Taber
Portrait of Sir Kenelm Digby, 1603-65 (Anthony van Dyck: From Wikimedia Commons, the free media repository)
In this post I discuss a historical scientific analogy used to discuss the distinction between animals and plants. The analogy was used in a book which is said to be the first major work of philosophy published in the English language, written by one of the founders of The Royal Society of London for Improving Natural Knowledge ('The Royal Society'), Sir Kenelm Digby.
Why take interest in an out-of-date analogy?
It is quite easy to criticise some of the ideas of early modern scientists in the light of current scientific knowledge. Digby had some ideas which seem quite bizarre to today's reader, but perhaps some of today's canonical scientific ideas, and especially more speculative theories being actively proposed, may seem equally ill-informed in a few centuries time!
There is a value in considering historical scientific ideas, in part because they help us understand a little about the path that scientists took towards current scientific thinking. This might be valuable in avoiding the 'rhetoric of conclusions', where well-accepted ideas become so familiar that we come to take them for granted, and fail to appreciate the ways in which such ideas often came to be accepted in the face of competing notions and mixed experimental evidence.
For the science educator there are added benefits. It reminds us that highly intelligent and well motivated scholars, without the value of the body of scientific discourse and evidence available today, might sensibly come up with ideas that seem today ill-conceived, sometimes convoluted, and perhaps even foolish. That is useful to bear in mind when our students fail to immediately understand the science they are taught and present with alternative conceptions that may seem illogical or fantastic to the teacher. Insight into the thought of others can help us consider how to shift their thinking and so can make us better teachers.
Analogies as tools for communicating science
Analogies are used in teaching and in science communication to help 'make the unfamiliar familiar', to show someone that something they do not (yet) know about is actually, in some sense at least, a bit like something they are already familiar with. In an analogy, there is a mapping between some aspect(s) of the structure of the target ideas and the structure of the familiar phenomenon or idea being offered as an analogue. Such teaching analogies can be useful to the extent that someone is indeed highly familiar with the 'analogue' (and more so than with the target knowledge being communicated); that there is a helpful mapping across between the analogue and the target; and that comparison is clearly explained (making clear which features of the analogue are relevant, and how).
Nature made engines
Digby presents his analogy for considering the difference between plants and animals in his 'Discourse on Bodies', the first part of his comprehensive text known as his 'Two Discourses' completed in 1644, and in which he sets out something of a system of the world.1 Although, to a modern scientific mind, many of Digby's ideas seem odd, and his complex schemes sometimes feel rather forced, he shared the modern scientific commitment that natural phenomena should be explained in terms of natural causes and mechanisms. (That is certainly not to suggest he was an atheist, as he was a committed Roman Catholic, but he assumed that nature had been set up to work without 'occult' influences.)
Before introducing an analogy between types of living things and types of engines, Digby had already prepared his readers by using the term 'engine' metaphorically to refer to living things. He did this after making a distinction between matter dug out of the ground as a single material, and other specimens which although highly compacted into single bodies of material clearly comprised of "differing parts" that did not work together to carry out any function, and seemed to have come together by "chance and by accident"; and where, unlike in living things (where removed parts tended to stop functioning), the separate parts could be "severed from [one] another" without destroying any underlying harmonic whole. He contrasted these accidental complexes with,
"other bodies in which this manifest and notable difference of parts, carries with it such subordination of one of them unto another, as we cannot doubt but that nature made such engines (if so I may call them) by design; and intended that this variety should be in one thing; whole unity and being what it is, should depend of the harmony of the several differing parts, and should be destroyed by their separation".
Digby emphasising the non-accidental structure of living things (language slightly tidied for a modern reader).
Digby was writing long before Charles Darwin's work, and accepted the then widely shared idea that there was design in nature. Today this would be seen as teleological, and not appropriate in a scientific account. A teleological account can be circular (tautological) if the end result of some process is explained as due to that process having a purpose. [Consider the usefulness as an 'explanation' that 'oganisms tend to become more complex over time as nature strives for complexity'. 2]
Scientists today are expected to offer accounts which do not presuppose endpoints. That does not mean that a scientists cannot believe there is purpose in the world, or even that the universe was created by a purposeful God – simply that scientific accounts cannot 'cheat' by using arguments that something happens because God wished it, or nature was working towards it. That is, it should not make any difference whether a scientist believes God is the ultimate cause of some phenomena (through creating the world, and setting up the laws of nature) as science is concerned with the natural 'mechanisms' and causes of events.
Read about science and religion
Two types of engines
In the part of his treatise on bodies that concerns living things, Digby gives an account of two 'engines' he had seen many years before when he was travelling in Spain. This was prior to the invention of the modern steam engine, and these engines were driven by water (as in water mills). 3
He gives a detailed account of one of the engines, explaining that the mechanism has one basic function – to supply water to an elevated place above a river.
His other engine example (apparently recalled in less detail – he acknowledges having a "confused and cloudy remembrance" ) was installed in a mint in a mine where it had a number of different functions, including:
- producing metal of the correct thickness for coinage
- stamping the metal with the coinage markings
- cutting the coins from the metal
- transferring the completed coins into the supply room.
These days we might see it as a kind of conveyor belt moving materials through several specialist processes.
Different classes of engine
Digby seems to think this is a superior sort of engine to the single function example.
For Digby, the first type of engine is like a plant,
"Thus then; all sorts of plants, both great and small, may be compared to our first engine of the waterwork at Toledo, for in them all the motion we can discern, is of one part transmitting unto the next to it, the juice which it received from that immediately before it…"
Digby comparing a plant to a single function machine
The comments here about juice may seem a bit obscure, as Digby has an extended explanation (over several pages) of how the growth and structure of a plant are based on a single kind of vascular tissue and a one-way transport of liquid. 4 Liquid rises up through the plant just as it was raised up by the mechanism at Toldeo
The multi-function 'engine' (perhaps ironically better considered in today's terms as an industrial plant!) is however more like an animal,
"But sensible living creatures, we may fitly compare to the second machine of the mint at Segovia. For in them, though every part and member be as it were a complete thing of itself, yet every one requires to be directed and put on in its motion by another; and they must all of them (though of very different natures and kinds of motion) conspire together to effect any thing that may be for the use and service of the whole. And thus we find in them perfectly the nature of a mover and a moveable; each of them moving differently from one another, and framing to themselves their own motions, in such sort as is more agreeable to their nature, when that part which sets them on work hath stirred them up.
And now because these parts (the movers and the moved) are parts of one whole; we call the entire thing automaton or…a living creature".
Digby comparing animals to more complex machines (language slightly tidied for a modern reader)
So plants were to animals as a single purpose mechanism was to a complex production line.
Animals as super-plants
Digby thought animals and plants shared in key characteristics of generation (we would say reproduction), nutrition, and augmentation (i.e., growth), as well as suffering sickness, decay and death. But Digby did not just think animals were different to plants, but a superior kind.
He explains this both in terms of the animal having functions that be did not beleive applied to plants,
And thus you see this plant [sic] has the virtue both of sense or feeling; that is, of being moved and affected by external objects lightly striking upon it; as also of moving itself, to or from such an object; according as nature shall have ordained.
but he also related to this as animals being more complex. Whereas the plant was based on a vascular system involving only one fluid, this super-plant-like-entity, had three. In summary,
this plant [sic, the animal] is a sensitive creature, composed of three sources, the heart, the brain, and the liver: whose are the arteries, the nerves, and the veins; which are filled with vital spirits, with animal spirits, and with blood: and by these the animal is heated, nourished, and made partaker of sense and motion.
[The account here does not seem entirely consistent with other parts of the book, especially if the reader is supposed to associate a different fluid with each of the three systems. Later in the treatise, Digby refers to Harvey's work about circulation of the blood (including to the liver), leaving the heart through arteries, and to veins returning blood to the heart. His discussion of sensory nerves suggest they contain 'vital spirits'.]
Some comments on Digby's analogy
Although some of this detail seems bizarre by today's standards, Digby was discussing ideas about the body that were fairly widely accepted. As suggested above, we should not criticise those living in previous times for not sharing current understandings (just as we have to hope that future generations are kind to our reasonable mistakes). There are, however, two features of this use of analogy I thought worth commenting on from a modern point of view.
The logic of making the unfamiliar familiar
If such analogies are to be used in teaching and science communication, then they are a tactic we can use to 'make the unfamiliar familiar', that is to help others understand what are sometimes difficult (e.g., abstract, counter-intuitive) ideas by pointing out they are somewhat like something the person is already familiar with and feels comfortable that they understand.
Read about teaching as 'making the unfamiliar familiar'
In a teaching context, or when a scientist is being interviewed by a journalist, it is usually important that the analogue is chosen so it is already familiar to the audience. Otherwise either the analogy does not help explain anything, or time has to be spent first explaining the analogy, before it can be employed.
In that sense, then, we might question Digby's example as not being ideal. He has to exemplify the two types of machines he is setting up as the analogue before he can make an analogy with it. Yet this is not a major problem here for two reasons.
Firstly, a book affords a generosity to an author that may not be available to a teacher or a scientist talking to a journalist or public audience. Reading a book (unlike a magazine, say) is a commitment to engagement in depth and over time, and a reader who is still with Digby by his Chapter 23 has probably decided that continued engagement is worth the effort.
Secondly, although most of his readers will not be familiar with the specific 'engines' he discusses from his Spanish travels, they will likely be familiar enough with water mills and other machines and devices to readily appreciate the distinction he makes through those examples. The abstract distinction between two classes of 'engine' is therefore clear enough, and can then be used as an analogy for the difference between plants and animals.
A biased account
However, today we would not consider this analogy to be applicable, even in general terms, leaving aside the now discredited details of plant and animal anatomy and physiology. An assumption behind the comparison is that animals are superior to plants.
In part, this is explained in terms of the plants apparent lack of sensitivity (later 'irritability' would be added as a characteristic of living things, shared by plants) and their their lack of ability in getting around, and so not being able to cross the room to pick up some object. In part, this may be seen as an anthropocentric notion: as humans who move around and can handle objects, it clearly seems to us with our embodied experience of being in the world that a form of life that does not do this (n.b., does not NEED to do this) is inferior. This is a bit like the argument that bacteria are primitive forms of life as they have evolved so little (a simplification, of course) over billions of years: which can alternatively be understood as showing how remarkably adapted they already were, to be able to successfully occupy so many niches on earth without changing their basic form.
There is also a level of ignorance about plants. Digby saw the plant as having a mechanism that moved moisture from the soil through the plant, but had no awareness of the phloem (only named in the nineteenth century) that means that transport in a plant is not all in one direction. He also did not seem to appreciate the complexity of seasonal changes in plants which are much more complex than a mechanism carrying out a linear function (like lifting water to a privileged person who lives above a river). He saw much of the variation in plant structures as passive responses to external agents. His idea of human physiology are also flawed by today's standards, of course.
Moreover, in Digby's scheme (from simple minerals dug from the ground, to accidentally compacted complex materials, to plants and then animals) there is a clear sense of that long-standing notion of hierarchy within nature.
The great chain of being
That is, the great chain of being, which is a system for setting out the world as a kind of ladder of superior and inferior forms. Ontology is sometimes described as the study of being , and typologies of different classes of entities are sometimes referred to as ontologies. The great chain of being can be understood as a kind of ontology distinguishing the different types of things that exist – and ranking them.
In this scheme (or rather schemes, as various versions with different levels of detail and specificity had been produced – for example discriminating the different classes of angels) minerals come below plants, which come below animals. To some extent Digby's analogy may reflect his own observations of animals and plants leading him to think animals were collectively and necessarily more complex than plants. However, ideas about the great chain of being were part of common metaphysical assumptions about the world. That is, most people took it for granted that there was such hierarchy in nature, and therefore they were likely to interpret what they observed in those terms.
Digby made the comparison between increasing complexity in moving from plant to animal as being a similar kind of step-up as when moving from inorganic material to plants,
But a sensitive creature, being compared to a plant, [is] as a plant is to a mixed [inorganic] body; you cannot but conceive that he must be compounded as it were of many plants, in like sort as a plant is of many mixed bodies.
Digby, then, was surely building his scheme upon his prior metaphysical commitments. Or, as we might say these days, his observations of the world were 'theory-laden'. So, Digby was not only offering an analogy to help discriminate between animals and plants, but was discriminating against plants in assuming they were inherently inferior to animals. I think that is a bias that is still common today.
Work cited:
- Digby, K. (1644/1665). Two Treatises: In the one of which, the nature of bodies; In the other, the nature of mans soule, is looked into: in ways of the discovery of the immortality of reasonable soules. (P. S. MacDonald Ed.). London: John Williams.
- Digby, K. (1644/2013). Two Treatises: Of Bodies and of Man's Soul (P. S. MacDonald Ed.): The Gresham Press.
- Taber, K. S. & Watts, M. (2000) Learners' explanations for chemical phenomena, Chemistry Education: Research and Practice in Europe, 1 (3), pp.329-353. [Free access]
Notes:
1 This is a fascinating book with many interesting examples of analogies, similes, metaphor, personification and the like, and an interesting early attempt to unify forces (here, gravity and magnetism). (I expect to write more about this over time.) The version I am reading is a 2013 edition (Digby, 1644/2013) which has been edited to offer consistent spellings (as that was not something many authors or publishers concerned themselves with at the time). The illustrations, however, are from a facsimile of an original publication (Digby, 1644/1645: which is now out of copyright so can be freely reproduced).
2 Such explanations may be considered as a class of 'pseudo-explanations': that give the semblance of explanation without actually explaining very much (Taber & Watts, 2000).
3 The aeolipile (e.g., Hero's engine) was a kind of steam engine – but was little more than a novelty where water boiled in a vessel with suitably directed outlets and free to rotate, causing it to spin. However, the only 'useful' work done was in turning the engine itself.
4 This relates to his broader theory of matter which still invokes the medieval notion of the four elements, but is also an atomic theory involving tiny particles that can pass into apparently solid materials due to pores and channels much too small to be visible.