Philosophical Letters by Margaret Cavendish is a peculiar text in which Cavendish engages with the views of some of the great philosophers and scientists of her time, but it is not a correspondence in which she communicates with those figures directly. Instead, she discusses a cross-section of their views with a third-person, a fictional “Madam.” Cavendish did succeed in having a small amount of back-and-forth correspondence with the leading intellectual men of her generation but there is no philosophical or scientific exchange between Cavendish and the four philosophers who are her prime “interlocutors” in Philosophical Letters – René Descartes, Thomas Hobbes, Henry More, and Jan Baptista Van Helmont. This chapter addresses an argument that a being depends for its social and political properties on the behavior of the beings that surround it, and more specifically that the status of an individual as a member of the philosophical community depends on the willingness of that community to regard the individual as authoritative.

This chapter argues that Cavendish holds that men and women are for the most part equal in their abilities and capacities. Some texts in her corpus present women negatively, but I argue that they all square with the more feminist view that appears in the passages that are predominant. Her non-fiction philosophical treatises, where she writes in her own voice, make clear that she embraces the view that differences in ability between women and men are due to surrounding societal conditions that are more favorable to men than to women. She then illustrates the view – and the sinews and joints of the societal conditions in question – in the imaginary worlds of her fiction. She transports women into scenarios that are as supportive to women as they are to men, and in which women flourish.

In Second Replies, Descartes draws a distinction between two methods of demonstration: analysis and synthesis. He nowhere offers a formal definition or account of the two methods, but he does make claims throughout his corpus, but especially in the Second Replies, that provide important clues as to the details of their nature. For example, he identifies analysis as a method of instruction, and he says that indeed it “is the best and truest method of instruction” and is the method that he employs in the Meditations. He says that synthesis “is very suitable to deploy in geometry” (AT VII 156, CSM II 111) and that it characteristically involves the presentation of a series of definitions, postulates, axioms, and theorems that together form a deductive chain of reasoning that forces even the most stubborn of minds to affirm its conclusion (AT VII 156, CSM II 110–11) (see deduction). He makes additional claims as well: that analysis is a version of a method that was highly regarded in ancient geometry; that it helps us to have clear and distinct perceptions of the primary notions of metaphysics; and that it is a method of discovery (AT VII 155–57, CSM II 110–12). He says that synthesis and analysis are complementary methods but one difference is that a successful analytic demonstration does not compel our assent (AT VII 156, CSM II 110–11).

Descartes draws a further distinction between the method of demonstration and the order of demonstration. Both analysis and synthesis must employ the proper order: claims that are put forward initially cannot depend for their support on claims that come later, and claims that are derived thereafter must depend solely on claims that have already been established (Gueroult 1984, 1:8–11). Descartes emphasizes that in the Meditations he tried to adhere to this order: in the First Meditation he refrains from affirming claims that are dubitable, and when he does finally stand behind metaphysical principles, they are either primary notions that are known through themselves or the conclusions of arguments whose premises comprised such notions (AT VII 155, CSM II 110). Any method must adhere to the proper order, and so it is in other respects that analysis and synthesis diverge.

Like many philosophers, Descartes draws a distinction between the existence of a thing and its essence or nature. Even if an entity does not exist, presumably there is still a fact about what the entity would be like if it did exist. Sometimes Descartes speaks of an essence as an idea that is before a mind when it is thinking of an object (AT VIIIA 27–28, CSM I 212–13; AT IV 348–50, CSMK 279–81). Descartes also speaks of the essences as things that exist independently of our ideas of them – for example, when he discusses existing objects and the essences that constitute them (AT IV 348–50, CSMK 279–81). For Descartes, an essence or nature can exist in thought, and it can also exist in reality or in re.

Descartes introduces the notion of a true and immutable nature in the Fifth Meditation. One of the central goals of this meditation is to provide an additional argument for the existence of God, and Descartes sets up that argument by engaging a discussion of truths about triangles and other geometrical figures (see ontological argument). More specifically, he notices that there are “ideas of things which even though they may not exist anywhere outside me still cannot be called nothing” (AT VII 64, CSM II 44). The reason they cannot be called nothing is that “various truths can be demonstrated of them” – for example, that the three angles of a triangle add to two right angles, and that the longest side of a triangle subtends the triangle's greatest angle. Such truths “are not my invention” for we “clearly recognize [them] whether we want to or not” (AT VII 64, CSM II 44–45). There is something that we are picking out when we demonstrate a result about a triangle, an entity that puts constraints on what we can truly think about triangles and what we cannot. Triangular figures may not exist, but the fact that we recognize truths about them entails that triangles are “not merely nothing” but “have their own true and immutable natures” (AT VII 64–65, CSM II 44–45).

Born at Gayhurst, Buckinghamshire, this Roman Catholic courtier with a reputation as an alchemist was a proponent of the view that physical phenomena can (and should) be explained only in terms of mechanistic qualities like size, shape, and motion. This is, of course, a Cartesian (and Hobbesian) view, but Digby (1644, 344, 346) claimed that it was also Aristotle's view and that the more traditional understanding of Aristotle (as offering scientific explanations that appeal to substantial forms) was a result of the distortions of the Scholastics. Digby believed that Roman Catholicism was the only guarantor of truth, but since he was persuaded that the mechanical philosophy of Descartes or Thomas Hobbes was correct, he decided that Aristotle's authority could be maintained only by interpreting it as an essentially mechanistic philosophy (Henry 2009, 43–75). Digby was active in intellectual circles, publishing the mammoth Two Treatises in 1644, among other tracts. He was an early member of the Royal Society, and over the course of his career he engaged with Hobbes, Robert Boyle, Marin Mersenne, Pierre Gassendi, Isaac Beeckman, Walter Charleton, and Descartes, among other important figures (Dobbs 1971, 25; Westfall 1958, 16).

The Two Treatises, Digby's landmark work, attempts to illustrate the power of mechanistic science but also aims to buttress the view that souls are immaterial. Digby's thought here is this: if all physical phenomena can be explained in terms of matter in motion, but certain aspects of rational souls cannot be so explained, then rational souls are not “physical phenomena.” If rational souls cannot be explained by matter in motion, then they are immaterial. Furthermore, if destruction or (in Aristotle's phrase) “passing away” is explained in terms of a dissolution and scattering of the particles of matter that constitute a thing, then souls must be immortal because, being immaterial, they are not constituted of material atoms (1644, preface).

Digby and Descartes (AT VIIIA 25, CSM I 210–11) agree that the existence of mental qualities cannot be explained in terms of qualities like size, shape, and motion and are thus the qualities of an entirely different substance.