Journal of the History of Philosophy

Are Corpuscles Unobservable in Principle for Locke? LISA DOWNING ARECORPUSCLESUNOBSERVABLEin principle for Locke? In this paper, I argue that the answer is no. l The question may seem small, but there has been a great deal of disagreement in the literature about it. Despite some controversy , the question has never been explicitly addressed in much detail. In this paper, I aim to resolve this controversy. Along the way, I hope to demonstrate that the issue of the observability of corpuscles is not a peripheral question about Locke's scientific views. Locke's philosophy, as I will argue, is seriously undermined by the assumption of a principled divide between unobservable corpuscles on the one side and observable ordinary objects on the other. Locke is committed to no such division; in I_~:ke's view there is no reason, in principle, why we might not someday observe corpuscles. The title question, of course, has only arisen since it has become a commonplace of Locke-interpretation that Locke was a corpuscularian. That is, Locke, in common with the most prominent natural philosophers of his day, held that physical phenomena are caused by the mechanical interactions of tiny particles which are characterizable in terms of a small number of "primary" qualities . In this he was particularly influenced by the corpuscular theories of Robert Boyle. The central tenet of Boylean corpuscularianism is roughly that all physical things are made up of tiny hard bodies of a certain size and shape. The macroscopic qualities of bodies are held to arise from these qualities of individual corpuscles," plus their arrangements and motions. It is nowadays ' So, they are observable in principle. Since, following Locke and his commentators, I concern myself especially with visual perception in what follows, in effect I argue for the stronger conclusion that corpuscles were v/sua//y observable in principle for Locke. ' Boyle at different times uses the word 'corpuscle' in two different, closely related, senses. 'Corpuscle' may either denote one of the smallest physically undivided particles, the minima naturalia, or a small, stable complex of them. On this see Peter Alexander, Ideas, Qualities, and Corpuscles (Cambridge University Press, 1985), 66. In what follows, I take note of this ambiguity [33] 34 JOURNAL OF THE HISTORY OF PHILOSOPHY 30:1 JANUARY 1992 rightly uncontroversial to assert that important aspects of Locke's philosophy (for example, his primary/secondary quality distinction and his conception of real essences) cannot be fully understood without taking note of Locke's corpuscularianism ; no sharp distinction between Locke's philosophy and his natural philosophy is possible or helpful.s Maurice Mandelbaum, whose influential essay "Locke's Realism" contributed much to this trend in Locke scholarship, holds that corpuscles have to be in principle unobservable for Locke.4 Peter Alexander, who centers his detailed interpretation of Locke upon Locke's corpuscularianism, argues for the same position, although his reasoning is unrelated to Mandelbaum's.5 Yolton 6 and Buchdahl7 take the opposing view. Woolhouse recognizes this as an issue, and explicitly remains neutral, s This question has also, naturally enough, troubled recent Berkeley scholars concerned with Berkeley's relationship to Locke and Berkeley's attitude towards corpuscularian scientific theories. R.J. Brook9 argues that corpuscles must be in principle unobservable; Daniel Garber '~ assumes that Locke believed corpuscles to be observable in principle. Most of Locke's interpreters who take my side of the dispute argue primarily from "microscopical eyes" passages (e.g., 2.93. i i and 1~)" that Locke must have believed corpuscles to be observable in principle. Although such passages are, of course, relevant (and I shall cite and employ them in a certain context), they are not entirely conclusive, and the main point of this paper is to take another tack. I want to distinguish two questions, which need to be addressed separately: only where relevant. I contend that in ne/thersense of 'corpuscle' were corpuscles unobservable in principle for Locke. s This is not, however, to claim that Locke's purpose in the Essay is to give a philosophical justification of corpuscular mechanism. Margaret Atherton takes issuewith the latter claim in her recent paper, "Corpuscles, Mechanism and Essentialism in Berkeley and Locke,"Journal of the History...