Having described in abstract terms what mechanisms are and how they figure in scientific explanation, I turn now to setting the stage historically for the contributions of cell biology. The project of identifying cell mechanisms began in earnest after 1940 in what was then unoccupied territory between cytology and biochemistry. Researchers were at best dimly aware that crucial cellular operations occurred in organelles for which no direct methods of investigation were available. These organelles were too small to be meaningfully examined with the light microscope and much larger than the reacting molecules in homogenates that biochemists prepared from broken cells.

New research tools, especially cell fractionation and electron microscopy, opened for investigation the uncharted territory between biochemistry and cytology. The goal was to explain how cells carry out their basic functions. In the nineteenth and early twentieth century, the activity that had received the most attention was cell reproduction. The basic operations in cell division, including those carried out by chromosomes in the nucleus, had been described by cytologists using stains and the apochromatic lens decades before the advent of cell biology. The focus of early cell biology was rather on functions performed in the cytoplasm, especially capturing energy and synthesizing proteins. The first important steps in developing mechanistic explanations of these functions was to identify the mitochondrion and the endoplasmic reticulum as the cell organelles responsible for each, a project largely accomplished in the 1940s (although the name endoplasmic reticulum was not introduced until the early 1950s).

REMOTENESS

From the opening lines of the Meteorology the science of nature emerges as a systematic investigation of the natural world. This investigation is systematic in the sense that it consists of an inquiry into the different parts of the natural world in the attempt to discover the explanatory connections existing between its parts. If this investigation is successful, it does not provide mere knowledge of the natural world; it provides understanding of it. But this investigation is systematic also in the sense that it consists in a study of the natural world in its entirety. While Aristotle does not insist on this point in the opening lines of the Meteorology, he is more explicit towards the end of PA 1. This logos ends with an exhortation to the study of the entire natural world: the celestial together with the sublunary world, and this latter in all its parts, plants and animals included (645 a 4–7). Aristotle takes it for granted that the natural world is constituted by a celestial and a sublunary part, and argues that the study of each of these two parts has its own appeal. In this logos, however, the emphasis is on the study of plants and animals.

This book develops the investigation I began in Corpi e movimenti: il De caelo di Aristotele e la sua fortuna nel mondo antico (Naples, 2001). There I discussed Aristotle's reasons for the view that the celestial bodies are made of a special body which naturally performs circular motion and is different from, and not reducible to, earth, water, air, and fire. I have also shown that very few in antiquity, even within the school of Aristotle, were prepared to accept this doctrine, though many, if not most of them, shared Aristotle's view that the celestial world is a special and somehow distinct region of the natural world. This book incorporates material from the Italian one but presents it in the light of a new project. By studying the reception of the view that the heavens are made of a special body, I have come to appreciate not only how unusual Aristotle's conception of the natural world is; I have also come to understand how this conception may have affected the way Aristotle conceives of the science of nature. This book is an attempt to explore the significance of the study of the celestial bodies for Aristotle's project of investigation of the natural world.

While Aristotle argues, against his predecessors, that the celestial world is radically different from the sublunary world, he is not envisioning two disconnected, or only loosely connected, worlds.

ARISTOTLE'S LANGUAGE

In antiquity it was common to refer to the celestial simple body as the fifth body, the fifth substance, the fifth element, the fifth nature, or even the fifth genus. This language strongly suggests that there was a tendency to think of the celestial simple body as an additional body. In all probability, from very early on, the view that earth, water, air, and fire were the simple bodies out of which the other bodies are made was largely accepted and relatively uncontroversial. Disagreement was confined to the existence of a celestial simple body. This body was perceived as an innovation whose need was not transparent to everyone and called for an explanation. In other words, from very early on, the scope of the debate was narrowed down to, and focused on, the need for another body alongside earth, water, air, and fire. At the beginning of the DC, however, it is an entirely open question how many simple bodies or elements there are. It is only in the course of the argument that Aristotle comes to the conclusion that there is a celestial simple body which is naturally moving in a circle along with four sublunary simple bodies which naturally perform rectilinear motion.

INTRODUCTION

Aristotle is not merely concerned with solving a list of problems or discussing a certain number of topics. He is engaged in an ambitious project of investigation. This project consists in an attempt to establish the right sort of connections – explanatory connections – between the things of the world. If this investigation is successful, it not only provides us with knowledge, but it gives us understanding of the world. The investigation of the natural world is no exception to this rule. Aristotle has left a certain number of logoi, each of which is a relatively independent and sufficiently self-contained argument devoted to a particular topic or problem. But there is no doubt that these logoi are conceived as parts of a unitary project of investigation. There is also no doubt that Aristotle has a certain understanding of the relations between these parts. This understanding is strongly dependent upon a specific conception of the natural world and the substantial assumption that this particular department of reality is, at least to some extent, intelligible to us. More directly, Aristotle is persuaded that the natural condition for human beings is to know and understand the truth, and that we can know and understand a lot about the natural world if only our investigation is conducted in the appropriate way.

NATURAL AND NON-NATURAL MOTIONS

The student of nature assumes the reality of the natural world and conceives it as a certain arrangement of natural bodies. Within the broad compass of natural bodies is found a remarkable array of bodies. They range from the living celestial bodies performing a circular motion around the earth, to the living sublunary bodies endowed with the capacity for poreia and displaying the maximum degree of bodily complexity (perfect bodies), to the stationary living sublunary bodies (inferior animals and plants), and finally to the inanimate sublunary bodies. The student of nature is concerned with all these bodies on the assumption that they are either simple or composite bodies. Composite natural bodies are themselves composed of natural bodies. Earth, water, air, and fire are the sublunary simple bodies. They are the ultimate material principles of all the bodies that we encounter in the sublunary world, including the artificial bodies.

All these bodies are liable to undergo motion from one place to another. Consider the case of a stone: if dropped from a hand, a stone falls downwards. But why? Aristotle's view is that a stone is composed of earth, water, air, and fire in a certain ratio, and earth so predominates as to impart its own natural downward motion to the stone.

BODIES AND MAGNITUDES

By this point I hope to have established that Aristotle sees his science of nature as a systematic whole. It should also be clear that this science is seen as a systematic whole because it presents an account of a world that is similarly systematic. More directly, and more boldly, the science of nature mirrors the system of nature. Aristotle's conception of the natural world follows from the research program conducted in the science of nature. In other words, it is the study of the celestial and sublunary bodies that leads him to believe that the natural world is a causal arrangement of a certain type, and to the view that the study of the celestial world should precede, rather than follow, the study of the sublunary world. In the first two books of the DC are collected the results that Aristotle reached in the study of the celestial world. In the following chapters, I shall focus on specific parts of the DC and show how unusual Aristotle's conception of the celestial word is, especially if it is considered in its historical context in relation to his predecessors and successors.