In his writings, Hobbes described Galileo as “the greatest philosopher of all centuries,” who had disclosed the nature of motion and thereby inaugurated a new physics. Although past scholars have frequently drawn attention to Hobbes's intellectual debt to Galileo, the book under review is the first monographic study devoted to the relation between the two thinkers. During his third grand tour of Europe (1634–36), Hobbes met both Galileo and Mersenne, who, according to Baldin, were the main sources of inspiration for Hobbes's mechanistic natural philosophy and whose names figure prominently in Hobbes's autobiographies and correspondence. This explains why Baldin devotes chapter 1 to Mersenne and Hobbes, before turning his attention to Galileo's influence on Hobbes's method, theory of matter, and science of motion.

Chapter 1 offers a comparison between Mersenne's and Hobbes's respective views on the epistemological status of the mixed sciences, on the relation between mathematical truths and physical hypotheses, and between experiments and ratio. Baldin also stresses some interesting analogies between Mersenne's and Hobbes's explanations of physical and astronomical phenomena, focusing in particular on their views on the propagation of sound and light. While Hobbes met Galileo only once, in the decade that he spent in Paris (1640–51), he frequently joined Mersenne's circle. He probably had access to Mersenne's library, which—as Baldin recalls—also contained copies of Galileo's Assayer and Two New Sciences. Chapters 2–4 document the impact on Hobbes's thought of these two works, as well as of the Dialogue and of Mersenne's paraphrase of Galileo's Mecaniche.

In chapter 2, Baldin argues that the influence of the Assayer is particularly evident in Hobbes's use of the metaphor of the book of nature as well as in his distinction between the real qualities of bodies (figure and extension) and the sensible qualities resulting from the action of the perceived body on the perceiving subject. As for the Dialogue, which, contrary to the Assayer, is explicitly mentioned in Hobbes's works, Baldin focuses on two intertwined issues—namely, Hobbes's and Galileo's objections to Kepler's theory of magnetic attraction and Hobbes's reaction to Galileo's theory of tides. The chapter also deals with some aspects of Hobbes's philosophy that were less directly influenced by Galileo, such as his account of causality in the De Motu, Loco et Tempore and in the De Corpore, and his classification of sciences in the Leviathan. Chapter 3 sheds light on conceptual and lexicographic analogies between Galileo's and Hobbes's physics. Baldin shows the crucial role that Galileo's principle of inertia and his concept of impetus play in Hobbes's De Motu, Loco et Tempore. He also documents the influence of the Galilean notion of momentum on the evolution of Hobbes's notion of conatus and deals with Hobbes's theory of free fall and his speculations on the cause of gravity.

Chapter 4 analyzes the influence of Galileo's theory of matter as formulated in the first day of the Two New Sciences on Hobbes's treatment of the nature of fluids in De Corpore and in the Dialogus Physicus de Natura Aeris. Although Baldin speaks of a “great affinity” between the theories of the two thinkers, a number of important differences emerge from his comparison. Hobbes's views concerning the composition of bodies, his account of hardness and cohesion, and his explanation of the passage from the solid to the liquid state are in fact profoundly different from Galileo's.

The book contains no general conclusion, but ends with an appendix in which Baldin motivates his choice not to deal with Hobbes's Short Tract. One of the greatest merits of the book is that it sheds light on the evolution of Hobbes's thought on the issues under discussion and on the difficulties that Hobbes encountered in trying to account for physical phenomena such as terrestrial rotation, the tides, and the fluidity of bodies in mechanistic terms. However, Baldin's reconstruction of this evolution suffers from two shortcomings. One is that it sometimes downplays the influence of important thinkers who propelled Hobbes's thought in new directions. Baldin mentions, for example, a possible influence of Gassendi on Hobbes's notions of inertia and gravity, but is silent about the strong similarities between Gassendi's and Hobbes's reinterpretations of Galileo's theory of tides. The other defect is that Baldin handles the notion of “mechanical philosophy” as if it designated a monolithic whole, thereby neglecting the tension, to which Dijksterhuis and Westfall pointed long ago, between the ambition to explain the hidden corpuscular mechanisms behind phenomena and the attempt to describe them instead through exact mathematical laws. It is precisely this tension that, in my view, explains the discrepancies between mathematical descriptions and physical explanations that are so frequently present in Hobbes's physics.