There is a new edition of Toland's (1704) Letters to Serena, edited by Ian Laesk. It is the first modern, English language edition. (I have not read it yet.) In order to celebrate this great news, I want to point to one non-trivial, generally overlooked feature that helps to explain the enduring significance of Toland's Letters (for other features, recall here, here, here, and here).
The official aim of the fourth (out of five) of Toland’s rhetorically complex Letters is to show that Spinoza’s “system is without foundation.” While there may be “incidental truths” to be found in Spinoza’s system, it is “false” (Letter 4.4, p. 135 [I quote by letter, paragraph, and page-number). In particular, throughout the fourth letter, Toland echoes Henry More’s criticisms of Spinoza[or so I claim] and argues that Spinoza’s account of motion and its relationship to matter is confused. Toland claims this criticism sets up his own positive argument, to be offered in the fifth letter that matter is “necessarily active as well as extended.” (Letter 4.17, 161; repeated in Letter 5.1, 164). In particular, he praises those mathematicians, which
“compute the quantities and proportions of motion, as they observe bodies to act on one another , without troubling themselves about the physical reasons of what every person allows, being a thing which does not always concern them, and which they leave to the philosophers to explain.” (Letter 5.9, 177).
To this claim Toland attaches without comment a quote in Latin from the scholium to proposition XI of Newton’s Principia. An innocent reader could not avoid thinking that Newton endorses Toland’s stance.
But in what follows, a fuller explanation will be given of how to determine true motions from their causes, effects, and apparent differences, and, conversely, of how to determine from motions, whether true or apparent, true causes and effects. For to this was the purpose for which I composed the following treatise. (Scholium to the Definitions; Newton's Principia, quoted from I.B. Cohen's & Anne Whitman's translation 1999: 413–14).[1]
Here I leave aside debates over Newton.
More important, according to Toland Newton, thus, teaches that mathematicians leave causal explanations to philosophers. This division of intellectual labor move Toland had already advocated in the midst of his criticism of Spinoza: “The Mathematicians generally take the moving force for granted, and treat of local motion as they find it, without giving themselves much trouble about its original [cause]; but the practice of the philosophers is otherwise, or rather ought to be so,” (Toland Letters 4.8 (141)).
So, by Toland’s light there is an intellectual division of labor such that “mathematicians” find “rules of motions” by “observations learnt from…experience.” (Letter 4.8, 140; this means for him "mathematicians" are what we would call 'empirical scientists'.) In practice, this means that they only deal with what he calls “local motion” (or a “change in situation”). From an analysis of local motion mathematicians can generate the “ordinary rules of motion” by “probable calculations. (Letter 4.8, 140) By contrast, philosophers assign causes and the ”principles” of “true” motion. Note two features of Toland’s proposed division of intellectual labor: (i) it is normative (“ought to be”); (ii) mathematical natural philosophers do not have last word on their own analysis.
So, while seeming to attack Spinoza by way of Newton’s authoritative example, Toland has limited the reach of mathematical explanations of the sort paradigmatically associated with Newton. To the Newtonians' horror, Toland then explains that if one understands that matter is essentially active[2] one does not need to appeal to God to explain the origin of the universe or to invoke a God of the gaps (Letter 4.15, 157ff; Letter 5 is full of criticisms of Newtonian commitments).[1] The best recent defense of the instrumentalist reading is McMullin, E. (2001). "The Impact of Newton’s Principia on the philosophy of science." Philosophy of Science, 68, 279–310. (I have been critical here; recall also.)
[2] On some understandings of the so-called ‘conatus doctrine,’ Spinoza’s position also implies that matter is essentially active. This is how Diderot took Spinoza, for example (see Wolfe). This is not the place to explore how Toland ultimately understood his own position in relationship to Spinoza. It is a much debated question to what degree Newton (as opposed to Clarke) allows the activity of matter—in the preface to the second edition of the Principia, the editor, Cotes, leaves no doubt that attraction (and, thus, activity) is essential to matter, but in print Newton remains agnostic on the matter in his lifetime.
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