In the Scientific Revolution the concept of body evolved along several divergent lines, from conceptions that rely exclusively on extension and motion to more elaborate accounts that include attributes such as solidity and force. A host of complications were disputed, such as atomism versus the infinite divisibility of bodies, the distinction between primary and secondary properties, and the possibility of a vacuum. This chapter explores these and other issues, but with an emphasis on the relationship between body and spatial extension. Descartes's three-part distinction—i.e., whether the relationship between body and extension is conceptually, modally, or really distinct—serves as a framework for investigating the development of early modern theories of material body, a process that laid the basis for the ontology and epistemology of modern science.

Already in the Rules for the Direction of the Mind (1628) Descartes formulated his fundamental theory of extension: “By extension we mean whatever has length, breadth and depth” (AT X 442, CSM I 58). These three dimensions are inseparable in reality: a surface or line is not really distinct from extension but only considered so by an abstraction of the intellect (AT X 446, CSM I 61). And the difference between them is merely “nominal” (AT X 449, CSM I 63). We can speak of countless other measurable “dimensions” of an extended thing, such as speed or number of parts. But these quantifiable modes of extension “add absolutely nothing to the things which possess them” (AT X 448, CSM I 62). Simply put, extension is three-dimensionality.

Anything extended is a body. Indeed, we should not say “body possesses extension” since we do not have different conceptions of body and extension: “We might just as well say … ‘that which is extended is extended’” (AT X 444, CSM I 60). In The World (ca. 1633), Descartes writes that extension is not a mere accident of body but “its true form and essence” (AT XI 36, CSM I 92). Later, in the Principles of Philosophy (1644), he formulates the same conception using the technical notion of attribute. Extension is the “principal attribute” of body: “Extension in length, breadth and depth constitutes the nature of corporeal substance” (AT VIIIA 25, CSM I 210). Unlike variable modes, attributes are merely “conceptually distinct” from the substance itself: we can have a clear and distinct idea of body apart from a certain shape or motion, but not apart from extension. The principal attributes thought and extension are “nothing else but thinking substance itself and extended substance itself, that is, mind and body” (AT VIIIA 30–31, CSM I 215). Furthermore, there is merely a conceptual distinction among the various attributes of a given substance. Thus, Descartes mentions that a body's divisibility into parts (AT IXB 53, CSM I 215 n. 1) and quantity (AT VIIIA 44, CSM I 226) are merely conceptually distinct from its extension (see distinction [real, modal, and rational]). This identification of body, extension, quantity, and divisibility is a crucial step in Descartes’ program to mathematize natural philosophy: “I recognize no matter in corporeal things apart from that which the geometers call quantity” (AT VIIIA 78, CSM I 247).

Descartes relied heavily on the idea of infinity, most crucially in his first cosmological argument for God's existence in the Third Meditation. He argues that my idea of God's infinity is not derived by negating my own finitude nor by extrapolating from my own limited intellectual growth; on the contrary, my perception of the actually infinite is prior to my perception of the finite and the merely potential. Moreover, this idea is “utterly clear and distinct” and contains maximal objective reality (AT VII 46, CSM II 31) (see clarity and distinctness). So it can only have come from a substance that contains infinite formal reality – that is, God (see being, formal versus objective).

God alone is strictly infinite. Yet there are created beings, like extension and numbers, which are unlimited in certain respects. So Descartes introduces a technical distinction between the infinite, “that in which no limits of any kind can be found,” and the indefinite, that in which “there is merely some respect in which I do not recognize a limit” (AT VII 113, CSM II 81; cf. AT V 355–56, CSMK 377). The distinction is ontological in the first instance; it concerns the degree of limitlessness in things. An ordinary body, for example, is limited in size but unlimited in divisibility. The extension of the universe beyond the earth and stars, which Descartes sometimes calls “imaginary space,” is unlimited in size but limited in power, intelligence, and the like (AT III 274–75, CSMK 166). But Descartes also emphasizes an epistemic side to the distinction. Infinite things are those that I “understand” to be absolutely unlimited (in all respects), while indefinite things are those in “which, from some point of view, we are unable to discover a limit.” Extension, for example, is indefinite because “no imaginable extension is so great that we cannot understand the possibility of an even greater one” (AT VIIIA 15, CSM I 202). But I understand God to be actually infinite “so that nothing can be added to his perfection” (AT VII 47, CSM II 32).

Ontologically, a body (Latin corpus; French corps) is nothing but extension, its principal attribute, just as a mind is nothing but thought (AT VIIIA 25, CSM I 210). As the famous wax experiment of the Second Meditation shows, a body is not the same as its color, smell, shape, or motion, but instead merely something “capable of being extended” in many different ways (AT VII 31, CSM II 21). Not even solidity, in the sense of felt hardness, pertains to the essence of body since a body would retain its nature even if it always receded from our touch (AT VIIIA 42, CSM I 224). Solidity in the sense of impenetrability does pertain to the essence of body since penetration implies co-location and co-location implies annihilation: “But what is annihilated does not penetrate anything else” (AT V 342, CSMK 372). Besides being extended, bodies are by nature divisible and quantifiable, but Descartes emphasizes that divisibility (AT IXB 53, CSM 1 215n1) and quantity (AT VIIIA 44, CSM 1 226) are themselves merely conceptually distinct from extension and hence from body itself. Finally, since our strict notion of space (what Descartes calls “internal place”) is also simply extension, it follows that “in reality the extension in length, breadth, and depth which constitutes a space is exactly the same as that which constitutes a body” (AT VIIIA 45, CSM I 227). Consequently, a vacuum or “empty space” is impossible (AT VIIIA 49, CSM I 229–30), and the body comprising the universe is unlimited and unified (AT VIIIA 52, CSM I 232). So “body considered in general,” or generic matter, consists simply in “being something which is extended in length, breadth and depth” (AT VIIIA 42, CSM I 224).

For the purposes of first philosophy, it does not concern Descartes that this austere, geometrical conception of body is far removed from the diverse phenomenology contained in our familiar experience of objects. And yet the purpose of natural philosophy is to explain the regular phenomena of nature from first principles and laws. So Descartes needs to bridge the gap between body in general and natural bodies. Unlike the parts of a purely geometrical space, there are intrinsic, persistent differences among natural bodies. And associated with these differences are diverse but regular behaviors.

Born in London, Bacon pursued a political career after legal education at Cambridge and Gray's Inn. He took a seat in Parliament in 1584 with ambitions for personal advancement and intellectual reform. Under James I, Bacon rose to lord chancellor but a bribery conviction ended his public service in 1621. He thereafter concentrated on his vision for a new system of learning. His major philosophical works are The Advancement of Learning (1605), the Great Instauration (including the New Organon) (1620), and the posthumous New Atlantis (1627). Thomas Hobbes (briefly Bacon's secretary) is the source of the famous story that Bacon died from an infection contracted during impromptu experiments in refrigeration.

An unfailing intellectual optimist and champion of innovation, Bacon derided the prevailing philosophies of science as conservative and authoritarian. He specifically criticized Aristotelian Scholasticism for its excessive abstraction, syllogistic method, and fixation with causes and Renaissance humanism for its bookishness, eclecticism, and Platonic metaphysics. At a more general level, Bacon maintained that scientific progress required emancipation from pervasive intellectual “idols” of the tribe (cognitive biases in human nature), of the cave (individual prejudices), of the marketplace (confounding of words and things), and of the theater (dogmatic adherence to authority).

As a metaphysical framework for natural philosophy, Bacon favored Greek atomism. On method, he advocated a thoroughgoing empiricism that begins with the meticulous collection and classification of directly observable facts. Laws or axioms are inductively generated from these experiments. Rather than “leap from sense and particulars to the most general axioms,” the disciplined philosopher “elicits axioms from sense and particulars rising in a gradual and unbroken ascent to arrive at last at the most general axioms” (2000b, 36). Bacon esteemed scientific knowledge not merely for its own sake but also for the “relief of man's estate” (2000a, 32) and was a proponent of natural religion: “Natural philosophy, after the word of God, is at once the strongest remedy for superstition and the most proven food for faith” (2000b, 75). The Royal Society was founded on Baconian principles.

While no inductivist, Descartes frequently echoed Bacon's call for careful observation in natural philosophy (e.g., AT VI 63, CSM I 143). The elevation of enumeration and synthesis over syllogism in the Rules for the Direction of the Mind and Discourse on Method suggests the influence of Bacon on Descartes’ early theory of method.

Descartes inherited from the Aristotelian tradition a model of reality comprising substances and affections or modes that inhere in substances. But he reduced the kinds of substance to just two and made the inherence relation strong enough to block what he considered the ontological excesses of Scholasticism. In a general sense, modes are the various ways of qualifying or conceiving of substances. We can speak or conceive of the duration, number, or shape of a given object; so all of these are its modes. In a more precise sense, only those features of an object that admit of variation are modes. In this sense, shape is a mode while duration and number are classified as “attributes” since a thing can change its shape but cannot cease to endure (or become more or fewer) without ceasing to exist (AT VIIIA 30, CSM I 214). And since God is absolutely immutable, he has no modes, only attributes (AT VIIIA 26, CSM I 211). Order, duration, and number may be termed “generic attributes” since they “extend to all classes of things” (AT VIIIA 23, CSM I 208). But there are also two “principal attributes,”thought and extension, which bifurcate the world of substance into two natural kinds: minds and bodies. The principal attributes have their characteristic modes: minds change as thought takes on various perceptions and volitions; bodies change as extension takes on various shapes and motions. And these modes account for everything we perceive in things: “Each substance has one principal property which constitutes its nature and essence, and to which all its other properties are referred” (AT VIIIA 25, CSM I 210).

Descartes would dismiss the question whether modes properly pertain to a substance or to its principal attribute since there is merely a conceptual, rather than a real or modal, distinction between a substance and its attributes (see distinction [real, modal, and rational]). That is, “we are unable to form a clear and distinct idea of the substance if we exclude from it the attribute in question” (AT VIIIA 30, CSM I 214). In reality, the principal attributes thought and extension just are minds and bodies: “They must be considered as thinking substance itself and extended substance itself, that is as mind and body” (AT VIIIA 30–31, CSM I 215). There is also merely a conceptual distinction among the various attributes of a single substance.