The traditional narrative of modern philosophy emphasizes the triumph, over the course of the eighteenth century, of Lockean empiricism over various forms of rationalism. But thinking in terms of a dichotomy between “rationalism” and “empiricism” obscures more than it clarifies. While it is true that thinkers following in the wake of Locke's Essay were forced to take the role of sense experience much more seriously than before, a salient feature of eighteenth-century philosophy was its exploration of just how the mind is able to gain understanding from experience. And this exploration often led philosophers to sharpen and refine innatist arguments, rather than reject them entirely, as Locke had done. Locke's attack on innatism thus helped to stimulate the emergence of a mature philosophy of common sense.

Although the term “common sense” had occasionally appeared in seventeenth-century philosophical discourse, it tended to be invoked in a rhetorical manner, as when Robert Ferguson stated that the first heretics of the Christian Church “overthrew common sense, and did violence to the Universal Uniform and perpetual Light of Mankind” or when John Ray said that the common sense of mankind tells them that animals suffer. But already in such appeals, the term functioned as a synonym for practical reason or commonly held principles or truths. During the eighteenth century, as “common sense” began to be widely used in the service of a growing egalitarian and democratic mindset, thinkers started to accord it more philosophical weight and substance. It was becoming increasingly important to reconcile elite knowledge with common understanding, particularly as middling classes assumed ever greater prominence in cultural and political life, and thus it should come as no surprise that the term assumed a new role in the vocabulary of philosophers during the eighteenth century.

Locke's Essay received many critical responses after its publication in 1689, and his rejection of innate principles was a central point of contention. As Yolton states, many people “found Locke's rejection of innate ideas, his treatment of conscience, and his dissociation of the law of nature from the inward principles commonly supposed to be inherent in man dangerously challenging to the established morality and to revealed religion.”

The previous chapter focused on the ways in which early-modern science, and the philosophy that developed in its wake, came into conflict with commonsense experience and understandings. However, it would be a mistake to think that the modern philosophy of common sense was simply a reaction to the New Science; in fact, modern science was both a stimulus and a methodological resource for thinking about “principles” of common sense. And there were other intellectual currents in the early-modern period—including the rise of modern skepticism—that led a variety of thinkers who predated Reid to argue that human beings intuitively perceive certain notions, ideas, truths, or principles that condition our experience and make moral, scientific, and religious knowledge possible. This tradition of thought stretched from Herbert of Cherbury and René Descartes through the Cambridge Platonists and other English innatists to Henry Lee, G. W. Leibniz, the Third Earl of Shaftesbury, Claude Buffier, Frances Hutcheson, Joseph Butler, George Turnbull (Thomas Reid’s teacher), and Henry Home (Lord Kames), Reid’s friend and interlocutor.

While lines of influence between such thinkers are not always evident, and they did not all employ the term “common sense,” they were responding to many of the same currents in modern thought, and in similar ways. Viewed against the background of such thinkers, the thought of Reid and his followers in the late eighteenth and early nineteenth centuries was a continuation and development of an existing, albeit loosely affiliated, tradition of modern thought. Reid was thus mistaken when he wrote in 1764 that the notion of “natural suggestions”— natural principles of mind like causality that are suggested by experience—had “entirely […] escaped the notice of philosophers.” Like most philosophers, Reid was trying to establish himself as someone with something new to say, and he mainly had in mind those post-Lockean philosophers who subscribed to the notion that the main constituents of thought are “ideas” produced by experience and reflection.

Thinkers have been pondering the nature of common sense, and its relationship to science and scientific thinking, for a very long time. In the ancient world, “scientific” knowledge (epistêmê in Greek, scientia in Latin) emerged as a counterpoint to everyday understanding and common opinion, until Aristotle produced a reconciliation of the two that set the course for scientific thought for the next two millennia. It was not until the early-modern period, when the New Science of Copernicus, Galileo, and Newton emerged triumphant, that common sense and its relationship to science again became problematic, remaining so to this day. This book is about this fraught relationship and about the early-modern thinkers who sought to address it, culminating in the thought of the philosopher Thomas Reid (1710–1796), the preeminent figure in the Scottish school of common-sense philosophy. It is a story full of fascinating twists and turns but is ultimately about the perennial quest to understand how the human mind is able to gain credible and reliable knowledge about the self, nature, other human beings, and God.

It is my contention that if we can understand the historical interplay between common sense and science, and the emergence of “common sense” as a contested term in scientific and philosophical discourse, we will have a better grasp of some of the fundamental and enduring problems besetting the relationship between science and society, that is, the problem of the public understanding of science. In some parts of the world, including particularly the United States, questions that have been long settled in the scientific community—for example, global warming, evolution by natural selection, the value of vaccines—remain controversial in the larger public arena. A disconcerting gap persists between everyday knowledge and understandings, and well-established scientific theories and facts. While there are many factors—economic, religious, political—contributing to this state of affairs, it builds upon a mismatch between our everyday, commonsensical judgments and intuitions, and the discoveries and methods of modern science. This book helps readers to better understand the fundamental contours of this relationship and why common sense and science may not be at odds after all.

Historians of philosophy suggest that philosophic reasoning arose in opposition to the common sense of the multitude. The earliest Greek philosophers—lovers of wisdom and seekers of the truth that lay behind appearances—often cast themselves as those who went against popular opinions and everyday prejudices. The mysterious teachings of Heraclitus (540–480 BCE) and Parmenides (515–450 BCE) seemed to contradict what was known to everyone. Heraclitus suggested that very few would ever understand the logos or underlying order of the world, while Parmenides advanced cryptic arguments to the effect that change was only apparent, not real. Democritus (460–370 BCE) posited a world made up of tiny atoms that was quite different from the world as we experience it. In doing so, Democritus made a distinction between what would come to be known as “primary” and “secondary” qualities. The former are qualities inherent in matter itself, while the latter are the qualities of things as they appear to us. Such teachings, historians of philosophy suggest, “set philosophers against common sense in a most dramatic way.”

Plato (428–348 BCE) did much, in his writings, to distance philosophers from the common people and to differentiate true scientific knowledge (epistêmê) from the misguided and murky opinion (doxa) of the rabble. At the same time, however, he advanced the notion that knowledge of absolute truths is in some sense innate, requiring dialectical reasoning to be brought to light. Experience of the sensible world is misleading and cannot by itself provide us with scientific knowledge; there is an intuitive dimension to knowing. A variation of this idea (albeit with much more faith in sense perception) would form one of the core concepts of the modern philosophy of common sense.

According to Plato, Socrates held that the body, with its inaccurate and misleading senses, hinders knowledge.3 Sight, touch, hearing, and taste provide only confused, relative measures of things, not their essence. True knowledge is abstract, universal, absolute, rational, and divorced from sense experience. The most that the multiplicity of sense experience can do is arouse perplexity and provoke thought, causing the mind to seek the essential unities that underlie the flux of experience.4 It is the task of the philosopher to lead men on this upward journey out of the realm of sense experience to higher levels of wisdom.

There are a variety of ways in which one can view the thought of Thomas Hobbes (1588–1679), but for our purposes he was important because he advanced a thoroughly mechanistic and materialistic view of human nature, informed by the ideas of Descartes, Galileo, and others, that left little room for ineffable “spiritual” entities, free will, and innate first principles of mind that provide us with unmediated knowledge of an independent reality. Hobbes saw “man” as a purely material being driven by his passions and the need to survive (and hence as essentially solitary and selfish), and the world at large as being entirely constituted by matter in constant motion.

For Hobbes, “There is no conception in a man's mind, which hath not at first, totally, or by parts, been begotten upon the organs of Sense.” Sense is a “seeming” or “fancy” caused by an external motion or pressure on one of our organs, which gets communicated via nerves “and other strings” to the brain and heart and then produces a resistance or counterpressure that is directed outward to the original cause. In step with the emerging scientific distinction between primary and secondary qualities, Hobbes asserts that colors and sounds and other such qualities are not in the bodies that produce them but rather in us; their external cause is only matter in motion. The mind itself contains little, however, before experience. “There is no other act of man's mind, that I can remember, naturally planted in him, so, as to need no other thing, to the exercise of it, but to be born a man, and live with the use of his five Senses.” All other faculties that seem proper to man “are acquired, and encreased by study and industry; and of most men learned by instruction, and discipline […] For besides Sense, and Thoughts, and the Trayne of Thoughts, the mind of man has no other motion.”

Belief and knowledge are hence based on our physical experience of the world. Science consists in our ability to order our sense experiences through the proper use of language: scientific knowledge is at root “knowledge of all the Consequences of names appertaining to the subject at hand.”

This chapter gives an overview of the problems raised by the concept of motion in the period investigated – specifically, its apparent integration of Being and non-Being, and its combination of Time and Space. It then discusses the central notions employed in the present project: the criteria or standards established for philosophical inquiry (the principles of non-contradiction and of sufficient reason, and a criterion termed “rational admissibility”) and the roles of logic and mathematics in establishing natural philosophy. This chapter lays out broad outlines, systematic and historical, of the issues to be discussed in detail in the ensuing chapters, each of which will deal with one thinker or one school. Thus this opening chapter will serve as a first orientation for the project as well as a reservoir for consultation if questions concerning the basic concepts employed arise during the reading of the whole book.

Chapter 2 presents the challenge that Parmenides’s philosophy presents for a scientific treatment of motion and change. It lays out the criteria for philosophy that we find established in Parmenides’s poem under his particular interpretations: consistency, rational admissibility, and a principle of sufficient reason. A careful examination of his use of negation shows that negation for him is a separation operator that indicates the extreme opposite to the thing negated. The counterpart to this understanding of negation is a connection operator that expresses absolute identity. A further step explains how Parmenides’s operators and his criteria for philosophy make it impossible to give any account of motion and change. Finally, it is shown that the cosmology in the doxa part of Parmenides’s poem should be understood as his attempt to expound a best possible cosmology and its short-comings – the rationale being that if even the best possible cosmology cannot fulfil the criteria for philosophy, no one else’s cosmology needs to be considered.

In Aristotle’s Physics we find for the first time motion and speed implicitly measured in terms of time and distance covered, as the discussion of book VI, chapter 2 shows. Aristotle’s explicit account of measurement, however, which he gives in Metaphysics Iota and with which this chapter starts, understands measure not only as homogeneous with the measurand, but also as one-dimensional only. Accordingly, the explicit measure of motion is simply time in the Physics, as we see from examining Aristotle’s understanding of time as the measure and the number of motion. For a full account of motion and speed and a complete response to Zeno’s challenge, however, a complex measure is needed, one that takes account of both time taken and distance covered. The chapter shows that this is exactly what Aristotle implicitly develops in his Physics, when he compares motions of different speed and responds to Zeno’s paradoxes of motion. But it is not what he can accommodate in his theory of measurement.

Chapter 6 examines Plato’s introduction of mathematical structures in order to explain the natural world. It contrasts his ‘mathematical approach’ to nature with that of the Pythagoreans, and shows how his use of mathematics enables Plato to make motion intelligible in itself to a certain degree. For this, the idea of measuring motion in temporal terms is crucial. However, Plato’s treatment of measurement in the Timaeus does not include measuring the distance covered by a motion. And Plato’s treatment of time and space (the receptacle) as entities of fundamentally different status, taking time to be intelligible in a way in which space is not, prevents him from connecting time and space in an account of speed. It is shown that Plato instead reduces speed to the time a motion takes. The chapter finishes by spelling out the problematic consequences of this reduction – that it only allows for restricted comparability of different motions and that in certain cases it can lead into inconsistencies.

This chapter discusses the central question why Aristotle, in spite of having everything required to conceptualize a complex measure of speed in terms of time and space, did in the end not explicitly develop such a measure. It is first investigated whether contemporaries of Aristotle may have worked with such a complex measure of speed, and concluded that it cannot be found in either of the two thinkers most likely to have done so, namely Eudoxus and Autolycus. The second part of the chapter investigates what made Aristotle cling to a simple measure and suggests that there are mathematical and metaphysical reasons: metaphysically, Aristotle cannot explicitly accommodate a relation as a measure of motion, since relations are derivative and problematic for him; mathematically, the principle of homogeneity which derives from the realm of Greek mathematics makes it impossible to combine of different dimensions in a single measure in the way needed for measuring speed in a mathematically informed physics such as Aristotle’s.