This hardness is not that of a pit inside the soft flesh of a peach. It is that of a very tight knot at the center of a net. It is hard because it has to hold so many heterogeneous resources together. Of course, the heart is important for understanding the circulatory system of the  human body, but Harvey certainly did not make his famous discovery by considering the heart on one side and the blood vessels on another. The same is true for science studies. If one takes the content on one side and the context on the other, the flow of science becomes incomprehensible, and so does the source of its oxygen and nutriment, as well as their means of entering the bloodstream. What would happen if there were no fifth loop? The other four would die off at once. The world would stop being mobilizable; disgruntled colleagues would flee in all directions ; allies would lose interest; and so would the general public, after expressing either its shock or its indifference. But this death would ensue just as quickly if any of the other four loops were cut off. (Latour 1999:106-7)
To define an entity, one will not look for an essence, or for a correspondence with a state of affairs, but for the list of all the syntagms or associations into which one element enters. This non essentialist definition will allow for a considerable range of variations, just as a word is defined by the list of its usages: “air” will be different when associated with “Rouen” and “spontaneous generation” than when associated with “rue d’Ulm,” “swan-neck experiment,” and “germs”; it will mean “transport of life-force” in one case and “transport of oxygen and transport of dust-carrying germs” in the other; but the Emperor will also be different when associated by Pouchet with “ideological support of spontaneous generation to maintain God’s creative power” and by Pasteur with “monetary support of laboratories without any implication about the subject matters of science.” What is the essence of air? All of these associations. Who is the Emperor? All of these associations. (Latour 1999:161)
Just as historians are not forced to imagine one single nature about which Pasteur and Pouchet would make different “interpretations,” neither are they forced to imagine a single nineteenth century imposing its imprint on historical actors. What is at stake in each of the two assemblages is what God, the Emperor, matter, eggs, vats, colleagues, and so on are able to do. Each element is to be defined by its associations and is an event created at the occasion of each of those associations. This is true for the lactic acid ferment, as well as for the city of Rouen, the Emperor, the laboratory on the rue d’Ulm, God, and Pasteur and Pouchet’s own standing, psychology, and presuppositions. The airborne ferments are deeply modified by the laboratory on the rue d’Ulm, but so is Pasteur, who becomes Pouchet’s conqueror, and so is the air that is now differentiated, thanks to the eventful swan-neck experiment, into the medium that transports oxygen on the one hand, and the medium that carries dust and germs on the other. (Latour 1999:165)
LATOUR, Bruno. 1999. Pandora’s Hope: essays on the reality of science studies. London: Harvard University Press.