Prompted by a fruitful exchange on the Facebook, here’s some historical notes on the roots of thought experiments.
Perhaps the most important event in the history of thought experiments comes in the surprising form of a condemnation of a specific style of natural philosophy – the Condemnation of 1277, issued by Etienne Templar. This condemnation had as its focus a number of Aristotelian propositions, all of which were taken to impugn on the power of God to do whatever He wanted (so long as it didn’t involve a logical contradiction), which included God’s inability to create multiple and different worlds from the actual world, the necessity of all things that happen in the actual world, and constraints on God’s ability to move the heavens in a rectilinear way.
An immediate consequence of the Condemnation was a focus on God’s power for ‘natural impossibilities’ – events with conditions that were impossible from within the received Aristotelian physics. Out of this focus developed two senses of the hypothetical: one concerned with defending Aristotle, the other with moving past him.
The former can be most clearly illustrated by Albert of Saxony:
‘In the fourteenth century, for example, Albert of Saxony, an influential scholastic arts master and natural philosopher, assumed the truth of the eternity of the world and also the existence of a fixed quantity of matter in the world. From these assumptions, he concluded that over an infinite time, this limited quantity of matter would have to furnish bodies for an infinite number of souls. In the course of an eternal period of time, the same matter might serve as the human body of a number of different souls. On the day of resurrection, however, when every soul receives its own material body, a finite quantity of matter would have to receive an infinite number of souls. This was a heretical state of affairs, because one body – indeed every body – would have to receive more than one soul. Albert’s response to this dilemma was typical for natural philosophers who had to contend with theological restrictions. He explains that “the natural philosopher is not much concerned with this argument because when he assumes the eternity of the world, he denies the resurrection of the dead.” Albert simply dropped the inconvenient theological consequences from his discussion but retained the eternity of the world for the sake of the argument. By such appeals and devices, medieval natural philosophers could assume the truth of almost any condemned proposition, provided that they did not proclaim it to be categorically or philosophically true.’ (Edward Grant, ‘The Foundations of Modern Science in the Middle Ages’, p. 80-81)
The more significant sense of the hypothetical, however, turned on God’s power to do anything short of a logical contradiction. This had the effect of encouraging speculation about the world and the way it could be, even though it was ‘naturally impossible’ given Aristotelian physics. The key point to note here isn’t that Aristotle was overthrown or subverted. By using God’s power as a vehicle for speculative natural philosophy, an important meta-counterfactual began to come into view: that the world could coherently be conceived in non-Aristotelian terms. Counterfactuals ‘according to the imagination’ soon became a hallmark of late medieval thought:
‘Natural philosophers actually devised an expression to epitomize this approach when they referred to such counterfactuals as secundum imaginationem, that is, “according to the imagination. The Condemnation of 1277 played a significant role in generating counterfactuals. Many of the condemned articles compelled the acknowledgement of God’s absolute power to do whatever he pleased short of a logical contradiction. Examples of counterfactuals that were derived from the Condemnation of 1277 include the possibility of other worlds, vacua within and beyond our world, and the possibility that God might move our world with a rectilinear motion. In each of these examples, medieval natural philosophers sought to derive consequences within an assumed framework of Aristotelian physics, even though what they initially assumed was impossible in Aristotle’s system. What emerged were a series of interesting speculations, or, as we might say, thought experiments, in which certain Aristotelian principles were challenged and, to some extent, subverted.’ (p. 147-148)
From these counterfactuals emerged new conceptualizations of the world, many of which would be inherited by the early moderns and which would give impetus to the scientific revolution. By clearing the space for non-Aristotelian concepts of the world to be coherently conceived, the groundwork was laid for the assumptions that would drive the scientific revolution:
‘A significant natural impossibility that derived from the condemnation of 1277 involved Article 49, which made it mandatory after 1277 to concede that God could move the world rectilinearly, despite the vacuum that might be left behind. More than an echo of this imaginary manifestation of God’s absolute power reverberated through the seventeenth century. When Gassendi declared that “it is not the case that if God were to move the World from its present location, that space would follow accordingly and move along with it,” he was using the supernatural motion of the world as a convenient support for his belief in the absolute immobility of infinite space. As spokesman for Isaac Newton, Samuel Clarke (1675-1729), in his dispute with Leibniz, also defended the existence of absolute space when he argued that “if space was nothing but the order of things coexisting [as Leibniz maintained]; it would follow that if God should remove the whole material world entire, with any swiftness whatsoever; yet it would still always continue in the same place.” Finally, the power of counterfactuals is nowhere more impressively illustrated than in the principle of inertia, which Newton proclaimed as the first law of motion in The Mathematical Principles of Natural Philosophy (1687): “Every body continues [or perseveres] in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed upon it.” In medieval intellectual culture, where observation and experiment played negligible roles, counterfactuals were a powerful tool, because they emphasized metaphysics, logic and theology, the very subjects in which medieval natural philosophers excelled.’ (197)
Theologians, Inc. 