Pierre Gassendi's Philosophy and Science: Atomism for Empiricists (review)

In 1971, Richard S. Westfall described Pierre Gassendi as "the original scissors and paste man": an irrepressibly eclectic compiler whose Syntagma Philosophicum was published only after his death "when the author was finally beyond the possibility of adding and patching." Westfall here echoes Voltaire and Alexandre Koyré who, as quoted in Saul Fisher's preface, also dismissed Gassendi as a lesser light of seventeenth-century natural philosophy. But Fisher's study shows that, lesser light or no, Gassendi anticipated several modern ideas in epistemology and matter theory.

Fisher's subtitle, 'Atomism for Empiricists,' encapsulates Gassendi's mission to ground his reinterpretation of Epicurean atomism in an epistemology that rejected the Aristotelian and Cartesian demand for certainty and invoked instead a reliabilist theory of warrant. The challenge for Gassendi was to convince empiricists to believe in something that could not be directly experienced. The challenge for Fisher is to discern whether Gassendi succeeded.

In Part I, Fisher describes Gassendi's efforts to establish a constructive skepticism in which we assent to knowledge claims not because they are certain but because they are reliable. What we learn through our senses is reliable because our senses receive "corpuscular transmissions" from external objects. What arrives at our senses is only the beginning, however. Borrowing from Sextus Empiricus, Gassendi proposed two ways in which evident sense data could serve as signs to reveal non-evident states of affairs. A commemorative sign allows us to assert the presence of one thing when there is sensory evidence of another, if the two have always been found together in our previous experience. An indicative sign suggests a hidden state of affairs which must inhere for us to have perceived the sign. In one example, also taken from Sextus, the presence of beads of perspiration on the skin is an indicative sign of pores in the skin.

Fisher next examines Gassendi's theoretical writing on scientific reasoning, which makes use of a syllogistic method called 'regressus demonstrativus,' and then considers how Gassendi applied that method in his interpretation of Pascal's famous barometer experiments. Fisher finds that, in order to derive evidence for the existence of void from the experiments, Gassendi had to go beyond the regressus method and introduce the non-empiricist idea of hypotheses. Fisher points out that Gassendi accepted a principle of "scalar invariance," meaning that "physical laws governing the behavior of bodies are invariant to their scale" (156), and that he believed that this principle had already been borne out by microscopic observations. (Had he believed that the principle was also borne out by telescopic observations, would he have anticipated Newton?) Consequently, we have warrant to give provisional assent to hypotheses based on scalar invariance.

Fisher then turns to Gassendi's atomism itself. Here Gassendi had to explain how an atomistic matter theory produces the world as we know it within the constraints imposed by empiricism. Yet much of his theory has a rational rather than an empirical basis. For example, Gassendi proposed that atoms aggregate into moleculae and concretunculae, which in turn aggregate to form familiar substances. This remarkable anticipation of the molecules of modern chemistry allowed him to evade the critics who felt that the formation of complex substances from atoms was improbable. Yet it was made without any direct evidence of moleculae.

The most puzzling aspect of Gassendi's atomism was his assertion that atoms have the God-given power of self-motion, which violated the principle of scalar invariance and left Gassendi trying to explain the inertial motion of sensible objects in terms of the eternal motion of their constituent atoms. One wonders what consequences of retaining scalar invariance and allowing inertial motion of atoms were so dire that Gassendi had to abandon the scalar invariance on which he had relied to bolster hypotheses about the micro-world.

The life sciences, especially embryology, posed formidable challenges to mechanical philosophers. Gassendi, too, as Fisher makes clear, struggled...