leibniz's impossible request

After Newton published his Principia Mathematica, which outlined the cause of planetary motion (namely, gravity), one of Newton’s peers Gottfried Leibniz revealed a major criticism. Leibniz held that while Newton had provided a good description of the properties of gravity and its connection to planetary movement, he had not given a causal explanation for why gravity exists and why it behaves the way it does. To this, Newton responded that there is no need to provide a causal explanation of gravity because this cause cannot be deduced from the current phenomena observed. I hold that Newton was completely valid in his rejection of Leibniz’s criticism because forcing scientists to follow a (potentially) endless line of questioning hurts science: continuing to fixate on these questions might hinder scientific progress that can happen with the universal laws we have now. Further, science thrives on testable hypotheses because these tests provide an empirical foundation for theories that can be built off of these claims, and Newton would have been unable to provide a truly testable causal explanation for the phenomena observed.

Imagine that Newton had given an answer to the cause of gravity, and call this Cause A. Newton did not know what Cause A was, but thanks to Einstein, we do: the cause is embedded in Einstein’s theory of general relativity. However, we can also ask what the cause of general relativity is, and we can call this Cause B. This is a recursive process — we can always ask “why?” and it is extremely unclear where we should stop (Cause C? Cause D? What about Cause Z?). René Descartes believed that the “first cause” (also known as the “prime mover”) had its roots in God. This implies that we would need to keep searching for causes and can finally stop after finding the cause that was set into motion by God. However, to most contemporary philosophers, this is an arbitrary stopping point — even those who believe in God can always ask “why did God set this cause in motion above all others?” or even “why does God exist?” Because these stopping points are arbitrary, we are left with a chain of questioning known as an infinite regress that leaves us with endless speculation without a single falsifiable answer. We can have no scientific progress if we must answer every question about the causes of phenomena before applying or accepting the theories that describe them.

Also, an extremely important aspect of science is its focus on testable hypotheses, because these tests serve as an empirical foundation in order for more theories to be developed. At the time of Newton’s publication of Principia, there was not much information on the limitations of his theory of gravitation that we know today, such as the perihelion precession of Mercury’s orbit (where Mercury’s point closest to the Sun shifts over time) and gravitational lensing (the way light bends due to gravity). Newton and his contemporaries simply did not have the technology to observe any of these phenomena. However, our advanced satellite cameras, telescopes, space expeditions, and advances in precise measuring devices have helped us confirm the theory of general relativity. Limited by the technology available at the time, any kind of hypothesis that Newton could make would not be grounded in any kind of empirical evidence. Thus, we could not apply these hypotheses to the real world, and a major aspect of scientific progress can be defined as applying theories to the real world and seeing what new discoveries we can make about nature. The question of asking “what is the cause of gravity?” is a valid scientific question: science is also clearly concerned with causality (We can see this through Newton himself, who sought to discover the cause of planetary motion). However, it is not valid to assume that gravitation as a theory is invalid because Newton did not explain why gravity exists. This is because it becomes a hindrance to science when we hyperfixate on untestable scenarios (such as the theory of relativity when we did not have the proper resources to test it), taking time, resources, and energy away from the scenarios that can actually be tested. The field of science was greatly impacted and furthered by Newton’s theory of gravitation: for example, Newton’s laws were used to calculate the specific trajectory of the Voyager spacecrafts which went on to take detailed pictures of Jupiter and Saturn, which helped improve our scientific understanding of the formation of these planets. This might not have happened when it did if Newton himself had believed that his theory must be grounded in a cause before he chose to publicize it.

One potential counter argument to my claim may be that we can conceive of a reality in which Newton could, in fact, have come up with a causal explanation that would have been testable. For example, we can imagine that Newton had provided an explanation following Descartes’ theory that planetary movement and the gravity we feel on Earth is due to particles called corpuscles that move in vortices with no gaps between them, and which produce the centripetal motion we observe in planetary orbits. This may have been closer to a causal explanation than the one Newton provided, because it employs a mechanism of physical particles interacting with each other, which is more tangible than the theory of gravitation. This could have been tested by measuring the force acting on a ball as it falls to the ground, and comparing this with a model of all possible accelerations and forces of corpuscles acting on an imaginary ball. If the net acceleration in all directions experienced by the ball in the real world was equal to that of the imaginary ball, the theory would have been validated. While this is true, and we could conceive of a reality in which Newton proposed a testable theory, this is an extremely low bar for scientific testability and for causal explanations: Newton believed specifically that a causal explanation for a phenomena would have to be “derived from the phenomena,” meaning it required an explanation grounded in empirical evidence. However, the previous counterargument still would not fit his criteria for a causal explanation, because Newton did not have the technology necessary to test it, and would not have been able to come up with a rigorous way to collect the empirical data.