Reynolds here investigates the role of metaphors in the creation of scientific concepts, theories, and explanations, using cell theory as his primary case study. He explores the history of key metaphors that have informed the field and the experimental, philosophical, and social circumstances under which they have emerged, risen in popularity, and in some cases faded from view. How we think of cells—as chambers, organisms, or even machines—makes a difference to scientific practice. Consequently, an accurate picture of how scientific knowledge is made requires us to understand how the metaphors scientists use—and the social values that often surreptitiously accompany them—influence our understanding of the world, and, ultimately, of ourselves.
The influence of metaphor isn’t limited to how we think about cells or proteins: in some cases they can even lead to real material change in the very nature of the thing in question, as scientists use technology to alter the reality to fit the metaphor. Drawing out the implications of science’s reliance upon metaphor, The Third Lens will be of interest to anyone working in the areas of history and philosophy of science, science studies, cell and molecular biology, science education and communication, and metaphor in general.
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The Early History of Cell Theory: The cell as empty chamber, building stone, and elementary organism
The changing nature of the cell concept and of concepts immediately related to it — from the container (the "cells" of Robert Hooke's cork, 1667) to the contained (the Energide of Sachs, 1892); from Gallerte (Treviranus, 1816) or Schleim (Schleiden, 1838), with or without Körperchen (Purkinje, 1836) or Körnchen (Valentin, 1835), to Cytoplasma (Kölliker, 1867) and Kernplasma (Strasburger, 1879); from "sarcode" (Dujardin, 1867) to universal Protoplasma (Cohn, 1850); from the "substance glutineuse, simple et homogène" of Dujardin to the immensely complex heterogeneous system which we know today — should serve as a permanent warning against a belief in the fixity of concepts, or in their value at any moment in time, save as a means of communication, or rapid reference to the present state of knowledge — Laurence Picken (1960, 1).
This chapter provides a history of some of the early developments of the cell concept beginning in the seventeenth century and the cell theory in the nineteenth century up to the early twentieth century. This will be a highly selective history, focusing chiefly on the metaphorical language used to talk and think about those things we today so casually refer to as "cells." Because this will be a philosophical history, I beg the indulgence of professional historians of science, who will no doubt shudder at my lack of attention to important issues regarding material, technological, institutional, political, social, and other factors that are of course crucial to a complete understanding of the developments under discussion. My aim, however, is to illustrate that how to talk and to think about cells has always been just as important an issue as how to physically investigate them by means of material technologies and techniques. But before we begin our history of metaphors that have informed the cell concept and the cell theory, we must say a few words about what metaphors are and how they differ from similes and other related concepts.
2. Metaphors, similes, analogies, and models: a brief account
Metaphor and simile are figures of speech used to draw a comparison between two things. A simile typically employs the terms "like" or "as" and attempts to create a vivid image. For instance, "I slept like a log" or "She's as busy as a bee." In both cases it is understood that the intent is to assert that the two things in question are similar in some particular respect. The first suggests that while sleeping I was similar to a log in that I was silent and still. The second suggests that the person in question was as active as a bee flying rapidly and incessantly from one flower to the next. With a simile it is clear that the things being compared remain distinct and that there is no intent to assert an identity in all or even any essential features.
The Oxford English Dictionary defines a metaphor as "A figure of speech in which a word or phrase is applied to an object or action to which it is not literally applicable." In contrast to similes, metaphors make comparisons without using the terms "like" or "as" that would indicate the two things in question share only limited resemblance. The statement "Life is a highway," for instance, invites us to compare the respects in which our life is similar to a highway. Both have beginnings and ends; both have unexpected twists and turns; we may encounter roadblocks, etc. But metaphors also encourage us to think of two things, not just as similar in some superficial respects, but as identical in some deeper fashion. So while we all understand that life is not really identical to a highway, when we use the metaphor we do tend to think of life as a kind of journey, and it is this underlying identity that makes the metaphor effective. Likewise, when scientists say things like "Genes are the units of hereditary information transmitted from one generation of organisms to the next," they are using the term "information" to describe the material molecules of DNA that get passed from one generation to the next. They are not simply saying DNA is like or similar to information, they mean DNA is in some essential sense a form of information. Information was traditionally associated with language, either spoken or written, and because the metaphor of information has also been associated with the metaphorical description of DNA as a genetic "code" (with triplets of nucleotide "letters" serving as "codons"), it has become quite natural for us to think of genetics and other aspects of biology as involving forms or types of information.
The key difference, therefore, between a simile and a metaphor is that metaphors encourage us to think of two things not just as similar in some nonessential properties, but as identical in some important essential sense. Both are used in science to draw analogies and to facilitate analogical reasoning. This is the intellectual process whereby, on the basis of a perceived similarity between two objects or systems, we transfer our knowledge and understanding of one with which we are familiar, to another about which we are less familiar. Analogical reasoning is premised on the assumption that if two systems are similar in one or a few properties, they may also be similar in others yet to be discovered (Bartha 2013).
Philosophers of science have been very interested recently in the role played by models in scientific inquiry. Models are an important element in how scientists attempt to represent the world and understand how it functions. Models take many forms: physical, mathematical, pictorial-diagrammatic, and linguistic, and they also frequently (though not always) involve forms of analogical reasoning. Some analogical models, such as the billiard ball model of a gas or Sewall Wright's adaptive landscape model of geneotype fitness, can be expressed by either simile or metaphor. Many scientific models and theories have their origins in metaphor, e.g., Darwin's theory of natural selection or the electromagnetic wave model of light. The key point for now is that metaphors are a powerful aid to reasoning by analogy. Detailed discussion about how scientific metaphors work will be taken up in chapter 5. I turn now to the history of metaphor in the creation of the cell concept and the cell theory.
3. Origins of the cell concept
The term "cell" was introduced into the natural sciences by the English naturalist and polymath Robert Hooke (1635–1703) in his account of observations of nonliving and living matter with a compound microscope, the Micrographia (Hooke 1665). The book contains sixty chapters, each devoted to observation of a specific material (the last three actually concerning Hooke's telescopic observations of the moon and stars). Chapter 18, titled "Of the schematisme or Texture of Cork, and of the Cells and Pores of some other such frothy Bodies," describes his observations of sections of dead cork plant. Hooke speaks of the porous nature of the material: "I could exceeding plainly perceive it to be all perforated and porous, much like a Honey-comb, but that the pores of it were not regular, yet it was not unlike Honey-comb in these particulars" (113). (See figure 1.1.) Although many popular accounts assert that Hooke was led to describe these small structural units as cells because they reminded him of the small rooms occupied by monks in a monastery, the actual text reveals that Hooke was in fact making a comparison to the polygonal cells of beeswax. The cell of honeycomb is itself a metaphor likely drawn from comparison to the small rooms of monks, so that Hooke's biological cells are a twice-borrowed metaphor. In describing the porous nature of cork tissue, Hooke uses alternately the terms "pores," "boxes," and "cells."
Next, in that these pores, or cells, were not very deep but consisted of a great many little Boxes, separated out of one continued long pore, by certain Diaphragms ...
A pore, as defined by the Oxford English Dictionary, is a "minute opening in surface, through which fluids may pass," and from the quotation above it seems that Hooke intended to use the terms pore and cell interchangeably, so that a cell in his original conception may in fact be composed of several smaller "Boxes." However, his usage of the term appears inconsistent, as the following examples illustrate: Our Microscope informs us that the substance of Cork is altogether fill'd with Air, and that the air is perfectly enclosed in little Boxes or Cells distinct from one another.
... the whole mass consists of an infinite company of small Boxes or Bladders of Air ... (113–14)
The pores are likened to "channels or pipes through which the Succus nutritus, or natural juices of Vegetables are convey'd, and seem to correspond to the veins, arteries and other Vessels in sensible creatures" (114). Hooke comments on the extremely minute size of these cells, pores, or boxes, suggesting that they may be too small even to allow the hypothetical atoms of the ancient Greek philosophers to pass through.
The pith material contained within a feather quill is also described as being composed of "very small bubbles," each "Cavern, Bubble, or Cell" being "distinctly separate from any of the rest" (116).
In any case what does seem clear, whether he uses the term "cell," "box," "bubble," or "bladder," is that what grabbed Hooke's attention was a structural feature whereby the tissue was divided up into distinct spaces or units separated by a wall-like enclosure. Hooke's cell then was not our modern conception of the cell, for it was not intended to denote a living, physiological, or reproductive unit. Hooke was not attempting to articulate anything like a modern-day cell theory. He was not claiming to have discovered a universal principle of anatomy or physiology in plants, let alone in biology in general. He was merely describing a particular type of structure observable in some living material. Hooke's original notion of a cell, an empty space enclosed by solid walls, is an example of catachresis, the borrowing of an existing term to fill a gap in vocabulary (Soskice and Harré 1995, 303–4). He might have created a brand-new term for the structures in question. For instance, he might have called them "jexes," for the simple purpose of having some label by which to refer to them, and had he done so we can only speculate how things might have turned out differently.
Other philosophers or naturalists would take up Hooke's term and apply it to similar structures in other plants. For instance, in the late seventeenth century and throughout the eighteenth, a multitude of investigators armed with microscopes would describe similar structures in a variety of plant tissues. These spaces were variously called "cells," "bubbles," "bladders," "cavities," and "vesicles" by observers such as Marcello Malpighi (1628–94), Nehemiah Grew (1641–1712), Albrecht von Haller (1708–77), and Christian Wolff (1679–1754). Despite an agreement that these structures were to be seen in plant tissue, there was fundamental disagreement about whether these cells were positive entities in themselves or merely empty spaces or voids in an otherwise continuous material. In other words, it wasn't clear whether these cellular spaces should be considered as foreground or background. Was the cell a thing or the mere absence of things? Some compared the presence of these cells to the bubbles in a foam, the froth of beer, or the holes in lace, while others regarded them as real and distinct entities. Use of the term "cellular tissue" or "Zellgewebe" by eighteenth- and early nineteenth-century writers in relation to animal anatomy further confuses the issue, for this term was used not with the modern-day notion of cells as distinct units in mind, but to describe a web-like appearance in connective (areolar) tissue formed by a network of fibers (Wilson 1944; Baker 1948, 112–14).
4. Origins of a cell theory
The microscope was an ingenious bit of technology that provided unforeseen powers of observation for the curious minded, but it was insufficient for the creation of what we know as the cell theory. Even astute observers like Antoni van Leeuwenhoek (1632–1723), the discoverer of the minute world of the infusoria (ciliates and bacteria), refrained from proposing that his observations had uncovered a common unit or principle of structure underlying all living forms. Nor was the microscope entirely necessary for this purpose. For at this time many writers were beginning to speculate their way toward philosophical theories of the nature of plants and animals and life in general. Georges Louis LeClerc de Buffon (1707–88) for instance suggested that plants and animals are composed of "little organized beings," which are themselves composed of primitive and incorruptible living atoms (Buffon 1749, 24), and the German Naturphilosoph Lorenz Oken (1779–1851) speculated that all animal flesh is composed of smaller Urthiere or infusoria (Oken 1805, 22). Many commentators have made the case that the theory of a universal principle of plant or animal structure was as much the result of prior philosophical ideas — chief among them atomism or corpuscularianism — as it was improvements in microscope technology. And yet it cannot be denied that improvements in the optical design of microscopes between the years 1830–40 to correct for spherical and chromatic aberrations significantly assisted efforts to identify an underlying and unifying principle of anatomy and physiology. Prior to these improvements, observations of minute elements in living tissue were disputed as artifacts confounded by imperfections in the lenses, halos of light being mistaken for "globules," for instance.
This early period of microscopical investigation also saw a diversity of alternate terms in circulation. While Stefano Gallini (1756–1836) used the term "cell" to denote a precise anatomical unit in 1792, others continued to employ the terms "bubble," "vesicle," "bladder," and "globule" (Dröscher 2014a). And in addition to these, the histologist Jan Purkyne (1787–1869) used the German terms "Körnchen" (little kernel or seed) and "Kügelchen" (little sphere) (Harris 1999, 86). So what advantage did the term "cell" have that would explain its eventual rise to dominance? I would argue that it was largely accidental. The cell concept does have certain merits, but as became clearer as microscopical investigation of animal anatomy and unicellular protozoa in particular advanced throughout the nineteenth century, the cell metaphor, with its suggestion of a clearly defined and rigid wall, is both inadequate and misleading as a universally applicable term. What the cell concept did provide was a useful search image for early synthesis-minded scientists who were looking to discover some unifying principle of design by which to arrange the plant and animal kingdoms. By emphasizing a unit surrounded by a distinct wall or boundary, the cell concept helped to focus the attention of investigators while they observed various specimens under the microscope. Of course the same can be said of all the other terms mentioned, insofar as they all highlight a discrete unit or entity. Harris (1999, 86f) mentions that whereas cellula, from which we get "cell," is Latin for an empty interior into which things can be put, the German equivalents for korn or kernel favored by people like Purkyne and his students, denotes a solid body from which living organisms develop.
For whatever reasons, "cell" — or rather the German equivalent "Zelle" — was the term used by the botanist Matthias Schleiden (1804–81) and his zoologist colleague Theodor Schwann (1810–82), and it is to them that the first articulation of the cell theory is most frequently credited. Schleiden, like many others at this time, was interested in identifying a fundamental unit of plant biology, a search that involved arriving at a proper conception of the plant individual. For Schleiden, who believed like many others that plants are compound organisms aggregated from simpler units, this was the individual cell (Elwick 2007). In many types of plants, well-defined cell walls are reasonably easy to identify with a microscope, especially if one is looking for them (a charge made by later critics of the cell theory). Schleiden also capitalized on the visible presence of the nucleus–which had been described by Robert Brown (1773–1858) in 1833 — to help identify individual cells in the composition of plant tissue. But Schleiden was not primarily looking for a structural or anatomical principle common to all plants; he was looking for a physiological and developmental unit or individual (note that the title of his monograph is Contributions to Phyto-genesis). As he wrote in 1838,
The idea of an individual, in the sense in which it occurs in animal nature, cannot in any way be applied to the vegetable world. It is only in the very lowest orders of plants, in some Algae and Fungi for instance, which consist only of a single cell, that we can speak of an individual in this sense. But every plant developed in any higher degree, is an aggregate of fully individualized, independent, separate beings, the cells themselves. Schleiden (1847, 231–32)(Continues…)
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Table of Contents
Chapter 1. The Early History of Cell Theory: The cell as empty chamber, building stone, and elementary organism
Chapter 2. Biochemical Conceptions of the Cell: From bag of enzymes to chemical factory
Chapter 3. Cell Sociology: The cell as social agent
Chapter 4. Cell Signaling: The cell as electronic computer
Chapter 5. Metaphors in Science: “Perspectives,” “tools,” and other meta-metaphors
Chapter 6. The Instrumental Success of Scientific Metaphor: Putting the scientific realism issue into perspective