Hume's Natural Philosophy and Philosophy of Physical Science

Matias Slavov

• Publisher: Bloomsbury Academic
• Year: 2020
• ISBN:
  9781350087880
• Edition: 1
• Topic: Philosophy
• Subtopic: Modern Philosophy

This book contextualizes David Hume's philosophy of physical science, exploring both Hume's background in the history of early modern natural philosophy and its subsequent impact on the scientific tradition. Drawing on Cartesian cosmology and Einstein's special relativity, and taking in topics including experimentalism, causation, laws of nature, metaphysics of forces, mathematics' relation to nature, and the concepts of space and time, this book deepens our understanding of Hume's relation to natural philosophy. It does so in addition by situating Hume's thought within the context of other major philosophers and scientists, including Descartes, Locke, Boyle, Kant, Newton, and Leibniz. Demonstrating above all Hume's understanding of the fluid relationship between philosophy and science, Hume's Natural Philosophy and Philosophy of Physical Science will provide new insights for historians and philosophers of science.

1

The Concept of Natural Philosophy

This chapter focuses on the concept of natural philosophy. I provide both a systematic and a historical account of what natural philosophy is. With regard to the historical account, I limit myself to explicating how natural philosophy was understood in the early modern period. A critical aspect in defining natural philosophy is to grapple with the relation of philosophy and physics.¹ The two are different, but a difference does not imply a dichotomy. I believe there to be a gray area between philosophy and physics. This gray area, in which the two domains of investigation overlap, is the proper field for natural philosophy.²

My examination of natural philosophy in this chapter runs chronologically backwards. I wish to start by examining how we might think about the relationship between philosophy and physics today. I have a specific rationale for choosing a reverse chronological order. Philosophy and physics clearly make up different domains of research for us twenty-first-century people. This might lead us onto the wrong track right from the beginning. We could fail to see the common issues that both philosophy and physics touch upon. A systematic definition of the concept of natural philosophy brings down a putative dichotomy (but not a difference) between the two, and therefore enables us to see better that these disciplines have many common intellectual roots. I understand this systematic approach as being necessary for assessing the relevant history of natural philosophy.

Philosophy and Physics

Philosophy and physics are nowadays very different branches of study. I will attempt to explain this difference succinctly. To do this, I have to both simplify and generalize. Simplification and generalization are unavoidable because there are so many different kinds of philosophies, as there are also many sub-disciplines within physics.

Here is a very basic introduction. Philosophy is involved with dialogic argumentation, a priori reasoning, intuition, formal logic, and conceptual analysis concerning the most fundamental questions. Philosophers tackle issues such as what exists, and how we come to know of its existence. Physics, for its part, investigates quantifiable natural phenomena with mathematics, by means of observation and experimentation. Physicists solve problems involving, for example, the relations of matter, motion, time, and energy. The methods of study and the specific problems addressed are not the only differences among the two disciplines. There is a major institutional gap between the two as well. Both in the secondary school teaching and in the academia, philosophy finds its home in the humanities and the social sciences, whereas physics stands as the paradigmatic example of natural science. Due to increasing specialization since the nineteenth-century industrial revolution in Western societies, a renowned expert in one field might be a complete amateur in another. In explaining this social divergence, C. P. Snow ([1959]1998) famously stated that the humanists/social scientists and the natural scientists make up “two cultures” of their own.³

For this book, it is vital to examine the philosophy/physics dichotomy. This does not mean that philosophy and physics are the same thing. They are not, for the reasons—different methodologies, specific research questions, and social institutions—I sketched above. But differences do not equal a dichotomy. As this is such a crucial starting point for this book, I wish to expound on this point in detail.

Consider the following proposition, which is a variant of Newton’s second law of motion: “It is a law of nature that when an object is impressed by a force, it will change its state of motion.” There is no doubt that this is a scientific proposition.⁴ To test the proposition is to engage in physics. In its most generic form, the sample proposition can be expressed in terms of mathematics,

, where

is the force acting on the object,

is the total linear momentum, which is defined as the multiplication of the mass, m, and the velocity,

, of the object, and t denotes time. A modified version of the proposition is subject to a simple experiment. Assume that mass is constant, so we can deduce that the general proposition in the form of

is a definition of change in motion, that is, acceleration, so the proposition can be expressed as

, in which

stands for acceleration. Now we can create a scenario involving a low friction object that is pulled by variable weights, and measure the distances it covers in a given time under the influence of varying forces. Consequently, it can be shown, within a margin of error that the force exerted on the object is directly proportional to the acceleration produced, that is,

.

At first, we may gather that the sample proposition, “It is a law of nature that when an object is impressed by a force, it will change its state of motion,” is all about physics. Examining the proposition requires defining physical quantities, mathematical derivation, creating an idealized target system that neglects factors like friction and air resistance, carrying out the experiment for a number of times, and finally presenting the results to peers in a quantitative and a graphical form. The proposition is an item of rudimentary physics, but, all things considered, it is a part of science.

Given the former exposition of a proposition of physics, one might be left wondering: What on earth could philosophy have to do with any of this? Philosophical ambition of addressing foundational questions about what exists and how we come to know it seems to be as far away from physics as it gets.

Still, a perusal of the sample proposition reveals that one may adopt a philosophical perspective to it, too. The proposition mentions a law of nature. What is a law of nature? Are laws of nature contingent regularities, like constants of nature such as friction and air resistance, or are they something more; do they instantiate physical necessity? This leads us to the question of what kinds of modalities pertain to the world: contingency, physical necessity, and/or logical necessity? The proposition is expressed in causal terms, as it mentions that an object is “impressed” by a force. Are laws, then, causal? If they are, what objects or events described by the proposition should be considered as causes and effects? What are objects or events, precisely? What kind of criteria should our judgments concerning causes and effects satisfy? Should cause and effect be contiguous, temporally successive, and distinctly separable? If these requirements are not satisfied, should we then conclude that laws do not instantiate causation, but rather that laws enable us to predict the probable outcome of an isolated system? How is it possible to identify causation? Is it by means of observing regularities, or by considering counterfactual terms, or by manipulating salient variables? The proposition refers to the term force, but later theories in fundamental physics do not make a reference to Newtonian forces. In what sense, then, do forces exist? Are they merely fictitious entities that are useful for engineering purposes, or do they exist in some effective, non-fundamental way? Do scientific propositions express historically specific and ever-changing ways of making sense of the phenomena around us, or is science able to grasp the structure of the world as it really is in itself? Is there a deep incongruity of different world views between the old and the new scientific theories, or do new ones contain an element of the old ones? And what about the role of mathematics in physics: is it just a calculating device for making satisfactory predictions, or does nature itself have a structure that can be identified with the aid of mathematics? And, finally: should metaphysics constrain physics in deciding these questions, or should it be the other way around?

The former analyses of the interrelated semantic, epistemic, ontic, and metaphysical issues indicate that the sample proposition relates to philosophy. Although the proposition is doubtlessly a physical proposition, it is imbued with philosophy. Thinking about modalities, causation, the structure of reality, and our limits of knowing it is philosophical activity par excellence.

Again, my purpose is not to demonstrate that philosophy and physics are the same enterprises. They are not. But there is something like a gray area between philosophy and physics. In my understanding, this gray area is the proper field for natural philosophy. Defining this term is not a straightforward issue. It could be defined as an obsolete usage of the word physics. There is an element of truth to this. In the early modern period, natural philosophy was essentially understood as a discipline that is involved with issues that we now call physics (and which the early moderns themselves sometimes referred to as “physicks,” too). I shall analyze this historical aspect in the next section of this book. Before that, I wish to make the notion of natural philosophy clear from a systematic perspective. Here I shall rely on Lee Smolin and Roberto Mangabeira Unger’s (2015: 75–7) definition. In their account there are four key elements that natural philosophy is composed of. In the following, I provide my interpretation of Smolin and Unger’s stipulations.

First, the subject matter of natural philosophy is nature. This is different from philosophy of science, because its subject matter is science itself. Surely natural philosophy is interested in the nature of science, like the structure of scientific theories, scientific inference, and scientific practice. Natural philosophy is not miles away from the philosophy of science (which is why I wanted to include the term philosophy of science in the subtitle of this book). But this interest is based on the larger argument of what nature is like, given the constraints of our limited cognitive capacities.

Second, natural philosophy has a critical objective. It aims to dissociate the scientific discoveries of nature from the interpretation of scientific theories. It does this from a distance; natural philosophy is not a part of science in the sense that it carries out empirical research or devises hypotheses for testing new theories. Rather, natural philosophy explores the semantics, epistemology, ontology, and metaphysics that relate to what physics has to say about nature. This can be beneficial for a new science in its early stages.⁵ A good example is the formulation of special relativity in the early twentieth century. The creation of the theory involved a philosophical evaluation of the concepts of space and time.⁶ Although special relativity ensued from a critical reflection of nineteenth-century electrodynamic physics and the mathematics related to it, philosophy was of central importance to the conceptual revisions of the notions of space and time.

Third, although natural philosophy maintains some critical distance to science, it does not set forth views that are independent of or above science. Natural philosophy has an interest in figuring out what nature is like by paying close attention to what physics has to say about it. Natural philosophy does not unravel facts about the world that would be somehow deeper than what physics may accomplish. This makes natural philosophy deviate from aprioristic metaphysics (as championed by, for example, MacDonald (2005), Lowe (2011), and Fine (2012)), which holds that metaphysics can establish knowledge about reality that is more fundamental than a scientific description of it. However, natural philosophy comes close to a naturalistic metaphysics (as envisaged by, for example, Maudlin (2007), Ladyman and Ross (2007), and Maclaurin and Dyke (2012)) which emphasizes that there is a continuum between philosophy and the sciences.⁷

Fourth, as Smolin and Unger (2015: 77) put it, natural philosophy “intervenes in discussion of the agenda of natural science,” by attenuating “the clarity of the divide between a discourse within science and a discourse about science.” So natural philosophy has a critical task in figuring out the limits of scientific inquiry. It neither just blindly follows the results of the natural sciences nor claims to have superior knowledge of nature compared with scientific results.

The systematic definition of natural philosophy that I have made in this section is, I shall argue, also important for understanding early modern natural philosophy. In the aftermath of Newton’s Principia, philosophy and physics started to separate. Still, scholars of the period were very much in...

Table of contents

• Cover
• Half-Title Page
• Dedication
• Series Page
• Title Page
• Contents
• List of Figures
• Preface
• Acknowledgements
• Copyright Acknowledgements
• List of Abbreviations
• Introduction
• 1 The Concept of Natural Philosophy
• 2 Science of Human Nature and Natural Philosophy
• 3 Experimentalism
• 4 Laws of Nature, Causation, and the Ontology of Forces
• 5 The Relation of Mathematics to Nature
• 6 Space and Time
• 7 Hume’s Impact
• Notes
• Bibliography
• Index
• Copyright