by Scott Soames
But this isn’t the end of the story. Because the doctrine seems to suggest implausible conclusions—e.g., that roses would have been colorless had there been no observers, and that they would change color if our sensory systems changed—this view, some versions of which are still advocated, is no longer as widely accepted as it once was. There is no need here to try to decide whether colors are, or are not, secondary qualities in Galileo’s sense. The fact that the issue remains a live one in philosophy today illustrates the difficulty in drawing a clear line separating science from natural philosophy in the thought of Galileo and other great figures of the Renaissance and early modern period.8 There was no sharp line because philosophy and science were closely intertwined. It is not so much that philosophy was contributing to an independent enterprise known as science, as that substantial parts of emerging natural science were philosophy, and substantial parts of philosophy were, if not exactly science, at least scientific speculation.
Whereas one might, looking back from our perspective today, say that Kepler and Galileo were primarily scientists and mathematicians, and secondarily natural philosophers, the British philosopher Francis Bacon (1561–1626) was decidedly a philosopher of science, rather than a scientist or mathematician himself. An early opponent of Aristotelian physics and metaphysics in a country in which Aristotle’s influence on the philosophy of nature lasted longer than it did on the Continent, he emphasized the practical value of technological innovations such as printing, gunpowder, and the magnet, as well as their power to change the world. Advances of this sort were, he pointed out, more the product of looking directly at nature rather than of studying Aristotelian or scholastic metaphysics. A gifted writer, he favored some form of inductive process for studying it, without, it seems, being fully up to date on the blend of precise observation and mathematical sophistication of the best scientists of his day. Though one cannot credit him with original contributions to science or its methodology (his account of which was arguably less sophisticated than that of the thirteenth-century monk Roger Bacon [no relation] discussed in chapter 2), Francis Bacon did help create a climate of opinion favorable to those who made true scientific contributions.
Much more can be said about René Descartes (1596–1650), who was not only a renowned philosopher, but also a first-rate mathematician and a reasonably accomplished scientist. One of the period’s most influential thinkers, he is best known for his arresting mind-body dualism, his attempt to establish a maximally secure philosophical starting point—I think, therefore, I am—and his goal of developing a sound method for advancing human knowledge. Together, these aspects of his thought set philosophy’s agenda for the next two centuries. However, his intellectual achievements originated in mathematics and science.
Having received his degree from the Jesuit College of La Flèche in 1616, he shortly thereafter moved to the Netherlands, where he met and worked with the Dutch mathematician and natural philosopher Isaac Beeckman. During that time, he developed mathematical techniques for describing complex geometrical figures without resorting to compass and ruler constructions. In so doing, he laid the foundations of analytic geometry by inventing a way of using ratios between lengths to describe lines that allowed later mathematicians to replace geometrical figures with algebraic formulas in a coordinate system dubbed Cartesian coordinates. After finishing this work, he spent most of the 1620s in Paris. During that period, he discovered the sine law of refraction, which computes angles of incidence and refraction when light passes through different media, and he used this knowledge to explain why we see rainbows where we do. He also worked on optics, mathematically describing shapes of different lenses.
In 1629, he returned to the Netherlands, where he undertook two grand projects, one in metaphysical philosophy focusing on the existence of God and the nature of the human soul, and the other in systematic natural philosophy, in which he attempted to encompass “all the phenomena of nature, that is to say, the whole of physics.”9 His most famous work, Meditations on First Philosophy, which appeared in two editions, 1641 and 1642, grew out of the first, metaphysical, project.10 The second project, in physics and natural philosophy, occupied several later works appearing between 1644 and 1650.11 He described the relationship between the two projects as follows:
[T]he whole of philosophy is like a tree. The roots are metaphysics, the trunk is physics, and the branches emerging from the trunk are all the other sciences, which may be reduced to three principle ones, namely medicine, mechanics, and morals. By “morals” I understand the highest and most perfect moral system, which presupposes complete knowledge of the other sciences, and is the ultimate level of wisdom.12
It is notable that he uses the word ‘philosophy’ to stand for a comprehensive system incorporating all theoretical knowledge, ‘physics’ to stand for natural science, ‘medicine’ and ‘mechanics’ to stand for practical inquires based on science, and ‘morals’ to stand for the highest knowledge, which, though normative, rests on knowledge of all else. It is also noteworthy that he regarded philosophical metaphysics as the source of all systematic knowledge.
What Descartes called ‘metaphysics’ is really a combination of what we now call ‘epistemology’ (theory of knowledge) and ‘metaphysics’ or ‘ontology’ (an inquiry into the fundamental types of things that make up reality, and the relations holding among them). His central questions of metaphysics were How can we acquire knowledge?; What is there to be known—i.e., what is the nature of reality?; What exactly are minds and bodies and how are they related?; and Does God exist? In the first of his six meditations, he uses radical skepticism as a tool to establish a secure ground for all knowledge.
Stated in modern terms, his strategy of using radical skepticism to unearth absolute certainty goes something like this. We now know that the contents of our consciousness are determined by stimulations of neurons in our brain. Thus, we may think, it is theoretically possible for neurons of a brain preserved in a vat to be stimulated in a way that exactly stimulates the real-life experiences of a normal human being—even though the brain doesn’t interact with anything in its environment in the way we interact with things in our environment. How then do we know that we aren’t brains in vats? If we can’t know we aren’t, how can we know that other people or physical objects exist? After all, envatted brains don’t know those things. Since their “perceptual” experience, which (we may assume) is identical with ours, doesn’t provide them with such knowledge, it would seem that ours doesn’t either. Having reached this point, we can drop the pretense about brains as so much unknowable baggage. Perhaps there are no brains, bodies, or physical objects at all, but only an evil demon feeding us sensations that make us think otherwise. If we can’t rule this out, we can hardly be said to know the most ordinary things that we commonly take ourselves to know. Can we?
Descartes thinks we can. To show this, he must first arrest the slide into universal skepticism by identifying the absolute certainty, I think, therefore I am. Clearly, whenever I think that such-and-such is so-and-so I am thinking, whether or not what I am thinking (that such-and-such is so-and-so) is true. Thus, when I think that I am thinking, it must be true that I am thinking; I couldn’t be wrong about that. But then, since I can think only if I exist, I know that I exist. In short, I think, therefore I am.
Next one wonders, What sort of being is this thinker—a mind, a body, or a union of mind and body? Descartes has an answer. Since one can conceive of oneself as existing without any body of one’s own, or indeed without any bodies existing at all, he reasons that it must be possible for one to exist without the existence of any body.13 But surely, being a body is essential to anything that is a body, and having a bodily part is essential to anything that is a union of mind and body. From this it follows that nothing that is a body, or a union of mind and body, could possibly exist if no bodies did. Thus, Descartes reasons, he, the agent employing his method of radical doubt, is neither a body nor a union of mind and body. Rather, he
is a mind for which having the ability to think is essential. Repeating this process on our own, each of us may validly reach the conclusion “I am essentially a thinking being, distinct from my body.”
The next step is to prove the existence of God. The purported proof relies on the observation that we have the idea of an infinite and perfect being, the existence of which is not dependent on anything else. Descartes takes it to be obvious not only that he is too limited and imperfect to be the source of this idea, but also that it must come from an infinite, perfect, and ontologically independent God. Since this God, being perfect, is no deceiver, Descartes concludes that he can put to rest the idea that he is systematically deceived by the appearances of his senses, and, with this, go on to put his knowledge of the world on a firm basis.
His argument for the existence of God is reminiscent of the ontological argument given by Saint Anselm in the eleventh century. Many versions of the argument, all of them widely regarded as suspect, have been given throughout the centuries. One version, called “the modal version,” begins with the observation that we have the idea of an infinite, perfect, and completely self-sufficient being whose existence doesn’t depend on anything else. Being self-sufficient, such a being must exist necessarily, if he exists at all. Since that idea isn’t inconsistent, such a necessary being is coherently conceivable. To say this, the proponent of the argument continues, is to say it’s possible that God exists necessarily—i.e., there is a state the world could be in only if the claim God exists would be true no matter what state the world was in. Since we can’t now deny that there is such a possible state, it follows that God exists, no matter what state the universe is in. Either God exists, no matter what possible state the universe is in, or he couldn’t possibly exist. Since we know it is possible that he exists, he must actually exist.14
For Descartes, the fact that God is no deceiver means that we are not systematically deceived, but it doesn’t mean we are free from error. It is up to us to reason properly, to formulate hypotheses from which empirical predictions can be derived, and to accept those hypotheses only after their predictions have been confirmed by observational evidence. This was the essential point of contact connecting Descartes’s “metaphysics” with his scientific endeavors and his mechanistic philosophy of nature. In the latter, he sketched a unified account of celestial and terrestrial physics based on laws of matter in motion, which, in conception, though not in empirical and mathematical execution, anticipated Newton.
The physics envisioned by Descartes was one in which matter is infinitely divisible, having only the properties of size, shape, position, and motion (the laws of which are decreed and sustained by God). As in ancient atomism, all physical change results from the movement, combination, and recombination of particles of matter. All apparent instances of action at a distance, including gravity and magnetism, are explained away as the result of the movements and collisions of particles. For Descartes, these principles apply as much to the animal world as to nonliving things. Unlike humans, animals are taken to be purely mechanistic systems, devoid of reason, will, and conscious experience. Only we are partial exceptions to the otherwise thoroughly mechanistic system of nature. Nevertheless, Descartes realized that our bodies function in much the same way that the bodies of animals do. For him, this meant that human respiration, heartbeat, nutrition, and many ordinary activities—including walking, running, and reflexively responding to external stimuli—are purely physiological processes having nothing to do with the reasoning of our non-corporeal minds. One of his more complex theories of this sort involved our visual perception of size, shape, and distance, of which he gave a non-mentalistic account. In addition to the subtle and fascinating details of Descartes’s overall view, the point to notice is its comprehensiveness—science, mathematics, philosophy, and a bit of theology, pursued as a seamless whole.
Among the seventeenth- and eighteenth-century scientists and philosophers influenced by Descartes was the giant of the period, Isaac Newton (1642–1727). The preeminent scientist of his era, and one of the greatest of all time, his stunning scientific prowess included the philosophical ability to confront conceptual tangles and transform them into more tractable challenges. As an undergraduate at Cambridge, he studied Aristotle, logic, ethics, and physics. Prior to graduating in 1665, he read Descartes on philosophical method and naturalistic philosophy of nature, while also teaching himself mathematics and the astronomy of Kepler and Galileo. Newton spent the next two years away from Cambridge studying gravity and inventing the integral calculus. He returned in 1667 to Cambridge, where he shortly became professor of mathematics. Unable to find a publisher for his work on the calculus, he turned to optics, producing results that weren’t published until 1704, in his Treatise of the Reflections, Refractions, Inflections and Colours of Light. He returned to orbital astronomy in 1679, giving a short paper to the Royal Society in December of 1684, which he expanded to Philosophiae Naturalis Principia Mathematica and published in 1687.15
His basic gravitational law holds that every mass of any size attracts every other mass with a force directly proportional to the product of the two masses and inversely proportional to the square of the distance between their centers. With this simple idea, plus a few others, he was able to explain a remarkably wide range of phenomena—including the behavior of projectiles, the orbits of comets, the size of planets, their orbits and moons, the motion of our moon and its effect on the tides, and certain very slow changes in the positions of stars due to a cyclic wobbling in the orientation of the earth’s axis (the precession of the equinoxes).16 The accuracy and precision of his results commanded assent and suggested that he had found genuine laws of nature, even though his treatment of gravity as a force acting at a distance was perplexing, and hard to accept for those who considered it not genuinely mechanical.
Even Newton found gravity hard to understand. In a letter written in 1692 he says:
That … one body may act upon another at a distance through a vacuum without the mediation of anything else, by and through which their action and force may be conveyed from one another, is to me so great an absurdity that I believe no man who has in philosophic matters a competent faculty of thinking could ever fall into it.17
Still, it wasn’t that Newton denied the existence of gravity, or the explanatory role it plays. On the contrary, he had been driven to this otherwise counterintuitive idea by the powerful explanatory role it played in so precisely accounting for the observed data. What he denied was that he had gotten to the bottom of what it is. In 1713, he wrote:
I have not yet been able to discover the cause of these properties of gravity from phenomena and I feign no hypotheses.… It is enough that gravity does really exist and acts according to the laws I have explained, and that it abundantly serves to account for all the motions of celestial bodies.18
Newton’s first law states that every body not acted on by external forces will, if at rest, remain at rest, or, if not at rest, will continue its uniform motion in a straight line. Motion is movement from one point in space to another. Space, for him, was a Euclidean structure infinite in three dimensions, consisting of eternally existing points (locations). The distance between two points, which can be numerically measured, is the length of the straight line connecting them. Uniform motion of a body is movement the speed and direction of which remain constant. For Newton, this talk of speed requires the elapsed time between two arbitrary moments to be, like the distance between any two points in space, constant and numerically measurable. In short, Newton presupposes absolute space and time.
This is highly intuitive, but also deeply puzzling. We observe objects moving relative to other objects. Since we have no way of observing absolute motions of these objects, we seem to have no way of determining the absolute motions in terms of which Newton’s laws are stated. He recognized this.
Absolute and relative spaces are the same in figure and magnitude; [both are 3-D Euclidean] but they do not remain always nu
merically the same. For if the earth, for instance, moves, a space of our air, which relatively and in respect of the earth, remains always the same, will at one time be one part of the absolute space into which the air passes; at another time it will be another part of the same, and so, absolutely understood, it will be continually changed [despite being relatively at rest].19
These thoughts give rise to a philosophical conundrum. How, if our observations of spatial locations and movements through space are always relative to the positions of other objects, including ourselves, can we draw conclusions about the direction, the velocity, and the positions of objects in absolute space? More pointedly, why, given the observational opacity of locations and movements in absolute space, did Newton formulate his physical laws in terms of it? The answer, in part, is that he found this conception of space intuitively plausible. But that’s not all. He also identified an empirical phenomenon, a species of circular motion, that seemed to require it.
Newton’s second law states that the change made in motion of a body by a force exerted on it is inversely proportional to its mass and directly proportional to that force, along the straight line on which the force acts on the body. His key experiment brilliantly connecting this law to absolute space involved water in a spinning bucket (suspended from the ceiling by a twisted rope that spins the bucket as it unwinds).
[T]he surface of the water will at first be plain [flat] as before the vessel began to move, but after that the vessel, by gradually communicating its motion to the water, will make it begin sensibly to revolve, and recede … from the middle, and ascend to the sides of the vessel, forming itself into a concave figure.… [T]he swifter the motion becomes, the higher will the water rise, till at last, performing its revolutions in the same times with the vessel, it becomes relatively at rest in it. This ascent of the water shows its endeavor to recede from the axis of its motion; and the true and absolute circular motion of the water, which is here directly contrary to the relative, becomes known and may be measured.20
