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EMPIRICIST METAPHYSICS. Time for Empiricist Metaphysics

I have argued that in Newton’s hands all three questions of whether time is absolute or relative, true or apparent, and mathematical or common, become empirically tractable through a project such as the Principia. But what is the significance of this conclusion for the metaphysics of time ? After all, it is the demands of Newton’s project that lead to the adoption of an absolute, true, and mathematical time parameter: for the purposes of constructing a physics of matter in motion capable of addressing the question of geocentrism versus heliocentrism, it is absolute time rather than relative time (for example) that serves our needs. And it might be thought, therefore, that the results are restricted to the following form: “For the purposes of solving the system of the world, we must treat time as if it is absolute, true and mathematical,” and that this claim leaves the deeper philosophical questions concerning the nature and structure of time untouched. But this would be a mistake.

To see why, we need to situate Newton’s work in its philosophical context. Newton’s Principia makes contributions to mathematics, mechanics, astronomy, physics, and philosophy, and the text of the Principia should therefore be read in many ways. One of these is as a contribution to philosophy. I argue that we are right to incorporate Newton into the history of philosophy of time, not because his physics has implications for questions in philosophy of time that are to be drawn out by philosophers, but for his own direct contributions to philosophy of time through the conceptual distinctions that he makes and the methodology that he employs.

The most helpful philosophical context for these purposes is the work of Descartes. In Part II of his Principles of Philosophy, published in 1644, Descartes set out the framework of a project the goal of which is to provide an account of all the rich variety of the world as we experience it in terms of matter in motion, where that matter is characterized by a minimal set of properties (extension in Descartes’s case), “motion” is restricted to local motion (i.e., motion from place to place), and the parts of matter move and interact according to specified laws. Since Descartes’s laws are both spatial and temporal, space and time also fall under the scope of his project. It is this portion of Descartes’s philosophy that Garber (1992) labeled “Descartes’s metaphysical physics.” It is indeed a “metaphysical physics” in the following sense: it is intended to give an account of the nature, structure, and operation of the material world. The concepts it employs, of matter, motion, laws, and so forth, are not intended to be understood “as if” (i.e., serving an instrumental purpose relative to the project in hand), but as being metaphysically significant. To this metaphysical project, Descartes attached his rationalist epistemology of clear and distinct ideas.

Many of the important philosophical moves made by Newton are best understood as taking place within the framework of this project, and as responses to difficulties that Newton found in Descartes’s execution of that project. They should therefore be read as contributions to the metaphysics of matter, motion, space, and time, just as much as Descartes’s. What makes this project look so different in Newton’s hands is not the project itself, but the epistemology and methodology by which Newton sought to carry it out. Newton’s methodology presses us to make the most of the actual world we find ourselves in as an epistemic resource in pursuing the metaphysical project set up by Descartes, and this fastidious attention to empirical details is an expression of Newton’s empiricist epistemology. We are being offered an empiricist metaphysics, but a metaphysics no less. Thus, for the purposes of philosophy, it is important to read Newton’s project as a continuation of Descartes’s, and to assess it on these grounds.

As an aside, just to be clear: this is not the classical empiricism that gets discussed in the chapter after Newton in the Blackwell Companion (2013) mentioned in the Introduction to this paper. In early modern philosophy, the common starting point for both rationalists and empiricists is “ideas,” which are cut free of any connection to the world; this generates the epistemic problem of how to connect them back up again, so that we can know anything about the world. Such a starting point is not neutral, of course, and it is not one that Newton accepts. He is an empiricist, in the sense that all our knowledge comes through our experience; but, for Newton, to know the world through our experiences is not to first know our “experiences” and to then know the world. He is clear that this is a mistake, and he is clear about the methodological implications that follow.

Challenges to the significance of Newton’s work for metaphysics should take place within this philosophical context. For example, one could challenge the value of Newton’s contribution to metaphysics by rejecting the goal of Descartes’s metaphysical physics (of giving an account of all the rich variety of the world as we experience it) as being irrelevant to the metaphysics of matter, motion, space, and time. The level of detail at which Newton attends to the actual world could perhaps make this move tempting, but I think it would be a mistake. Unless we take the actual world as our guide, we risk constructing a metaphysics of matter, motion, space, and time that fails to include the actual world among its possibilities. Newton’s methodology requires us to take very seriously the goal of including the actual world among the possibilities we are addressing, and the metaphysical project set up by Descartes is therefore to be carried out with careful attention to empirical details.

One could also challenge the framework of the project (in terms of matter, motion, and laws) or the methodology adopted for carrying it out. For those of an empiricist inclination, seeing the framework and the methodologies in action will be an important part of this assessment. In being allowed to prove themselves (or not) with respect to the goals of the project, they prove their utility (or not) with respect to the metaphysics of matter, motion, space, and time.

If we adopt this approach, then the framework and the method will be judged by their results, and the significance of the project for metaphysics will depend in part on how fruitful the project proves to be with respect to the goal of giving an account of all the rich variety of the world as we experience it. Failure to succeed with respect to the goal is a legitimate route by which to challenge claims about the significance for metaphysics of the conceptual investigations carried out within the project. Notice that this depends on assessing the detailed empirical success of the project, and this is characteristic of the empiricist metaphysics that I am describing in this paper.

My claim is that Newton’s Principia makes direct contributions to the metaphysics of space, time, matter, and motion, and that this becomes visible if we read the Principia as a contribution to philosophy, in its philosophical context, by beginning with our metaphysical questions concerning space, time, matter, and motion as they were prior to Newton, and seeing what happens to them through the process of Newton’s attempts to carry out Descartes’s metaphysical physics project. In particular, Newton’s results concerning time are not of the above “as if” restricted form, but are contributions to our investigations into the nature and structure of time, and can be read as offering an empiricist metaphysics. To see how we address general questions about the nature of time by means of such an empiricist metaphysics, we return to Newton’s absolute, true, and mathematical time. For each of the three distinctions that Newton makes, there are issues concerning the nature and structure of time that we are being asked to address, and for each of these Newton is providing empirical purchase such that, as we shall see, addressing these questions is no longer a matter of general philosophical reasoning, but depends on the details of empirical enquiry. For those of us who value history of philosophy as a route toward understanding the philosophical questions that we have today, we know how important it is that we tell and re-tell that history; the case of Newton on time is one example of how we also need to tell and re-tell the history of physics, read as a contribution to philosophy, in order to re-tell the history of philosophy.

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