A Bug's Life: Aristotle's Metaphysics of Divided Insects

The aim of this paper is to point out a latent dilemma that lurks in the final chapters of the treatise De Motu Animalium (hereafter, MA), which threatens to undermine the Aristotelian understanding of organisms and the usual boundaries between the natural and the artificial. The difficulty arises when we consider some of the passages of the treatise dealing with the way in which the limbs and organs of a living being take part in the life of the organism as a whole. In these paragraphs Aristotle advocates for a central organ (the heart), which is supposed to be the primary mover of the body, and rejects the supposition of multiple engines spread through the joints, that would be capable of setting into motion the extremities that depend on them.

As Martha Nussbaum has shown, the thesis of a decentralization of the motion principles is supported by the recurrent example of worms and other undifferentiated bodies that, on being divided, show a residual and apparently autonomous movement for a given time (; De Anima I 5, 411b22). ¹ In this brief period, everything happens as if each of the segments resulting from the dissection had its own inner principle of rest and movement . From a theoretical point of view, this behavior is unsettling, since it seems to confirm the relative independence of the divided segments with regard to a "center" that governs the animal and imparts movement to its different parts.

The challenge that the counterexample of the divided worms represents for the theory of a central cardiac mover leads Aristotle to design a thought experiment, which aims to neutralize the undesirable outcomes that seem to arise from the dissection. Appealing to the example of a stick moved by a hand (see MA 8, 702b6), ² Aristotle seeks to curtail the impression that each segment of a divided worm is fitted with an independent motion principle. He seems to fear that, if this entomological observation were applied to more complex and differentiated organisms, the appeal to a central mover could be easily replaced by the introduction of multiple local motors, each of them located at the starting point of a limb. In such a case, organisms could legitimately be taken as automata, insofar as their movement could be decomposed into that of their self-moving parts (for this suggestion, see Nussbaum 1978, 359).

The analogy of the hand and the stick fits the defense of a sort of "kinetic holism," which states that the animal has a single central mover, whose impulse is relayed to the remaining parts of its body, each of which is devoid of intrinsic movement (MA 10, 703a36-37; 8, 702b1-3). If this were not the case, one of the core intuitions of Aristotelian ontology would be jeopardized: that which contends that an actual substance never consists of other actually existing substances (Metaphysics Z 1039a3-14; hereafter Metaph.), so that an animated body cannot be made up of other bodies that move by themselves in virtue of their own fuvsi~. (Insects that can be divided into animated sections would be an obvious challenge to this law).

It seems reasonable to assume that Aristotle is deeply committed to the defense of such a central moving principle. Nonetheless, he faces the challenge of achieving that goal without weakening another equally crucial thesis of his ontological framework: that which opposes sensible objects with intrinsic dynamism to artifacts lacking fuvsi~. Although Aristotle must place the movement of the living being in a central organ that prevents it from disintegrating into multiple unconnected local motors, he should also maintain that the living being is composed of natural parts. The natural character of those organic parts seems to require that they not be totally passive, but rather that they move spontaneously, instead of being dragged along like a stick. We will try to show that when Aristotle deprives limbs of autonomous movement to transfer all dynamism to the heart—which becomes almost "a separate living being" (MA 703b21)—he runs the risk of equating hands and sticks, perhaps beyond his own levels of...