Philosophy of the Sciences (PHIL 2130)

Lecture 1: Thought Experiments

1. What is science? How can we distinguish science from non-science? This problem has intrinsic interest, but is also of political and educational importance. For we value science for its objectivity, its contribution to technological advance, its success in revealing the nature of the physical world, its explanatory power and we respect the methods used by scientists to acquire knowledge. So if some activity claims itself to be scientific, we need to examine its credentials to determine whether it deserves to be so called, deserves to be heavily funded, deserves to be taught in schools. We think of hypotheses and of experiments used to test them. Usually, when we think of scientific activity, we think of people in white coats making observations, usually aided by highly sophisticated equipment.

2. A thought-experiment is a creative processes in which we obtain a result not by the usual methods of constructing apparatus, running tests, observing and measuring, but by thought alone. A question then arises: Can there be thought-experiments in science?

3. There is a tendency to think of scientific practice as divided into theorizing and experimenting. Until quite recently, philosophers of science were mainly interested in the former, and experimental work was regarded as a low status occupation. Some modern writers (e.g., Ian Hacking, Representing and Intervening and Peter Galison, How Experiments End) have attempted to show that this is a misconception, and have given detailed consideration to experimental work, not least because such work often proves to be a stimulus to theorizing. In fact, one might argue that the division between scientific theorizing and experimenting is artificial. Thought experiments are an illustration of this point. Here experiments are not conducted in a regular laboratory, but in the `laboratory of the mind'; as in the construction of theories, what is required is imagination and pure thought. The apparatus for such experiments is created by the imagination.

4. Science and philosophy (particularly ethics) have been particularly fertile fields for thought experiments, and this fact is quite easy to explain. Sometimes, in science, we need to consider ideal situations - frictionless planes, perfectly elastic bodies etc.; these don't exist in the real world, but we can mentally create such fictions. Also, there are experiments which it would be physically impossible for an experimenter to perform - e.g. running along a light ray at the speed of light, and observing it -- but such experiments can be done mentally.

5. One question that we should keep in our minds as the discussion proceeds is why thought experiments work. This is a particularly acute problem in the field of scientific thought experiments. For here we seem to be able to obtain firm empirical results -- scientific laws -- without having to do any empirical work, without having to make observations or use measuring instruments. It seems as if we are `getting something for nothing'.

6. Before we look at thought experiments in science, let us briefly look at some examples in ethics and mathematics.

7. A famous thought experiment in ethics was invented by Judith Thomson. She wanted to show that abortion is morally permissible. Of course, the claim that abortion is morally permissible is very controversial, but Thomson thought that consideration of an imaginary analogous case would be persuasive. In her thought experiment, someone wakes up one morning to find that a famous violinist who is dying from a blood diease has been attached to your body and your blood circulation system connected to his, so that his life now depends on you. But, don’t worry, after 9 months of sharing your blood, the violinist will be totally cured, he can detached from your body, disconnected from your blood circulation system, and can go and lead a happy, independent healthy life. Thomson says that it would be generous of you to let the violinist stay attached to you for the 9 months, but you are not morally obliged to do so. And likewise (so Thomson wishes to say) a woman who does not plan to become pregnant, but nevertheless does so, is not morally obliged to sustain the foetus.

8. Consider next the thought experiment of James Rachels which is designed to show that the American Medical Association is unjustified in forbidding active euthanasia (mercy killing) while allowing passive euthanasia (letting a patient die). What is the moral difference between drowning your 6-year-old cousin and just standing by watching him drown after he has hit his head on the side of the bath, when, with no effort, you could rescue him? No difference -- hence there's no morally relevant difference between killing and letting die. The example is, I think, legitimate and persuasive, even though, ultimately, it may not work.

9. Now an example from mathematics: a thought experiment designed to dissuade someone from the belief that equal perimeters must enclose equal areas. We could imagine forming a closed loop of string first into one rectangle, then into others. We could say that the length of a rectangle is x, the breadth y, and the length of string is constant, a. So the question is: if 2(x + y) = a, is the product of x and y, xy, also a constant? We could establish the result using calculus, and it would be quite complicated. But imagine a loop of string 100 cm. long formed into a square. The enclosed area is 625 cm². Now imagine putting two knitting needles inside the loop and pulling them apart. The resulting shape is almost 50 cm. long by (say) 1/₁₀₀ cm. wide, making an enclosed area of 1/₂ cm² -- we don't need to measure, we can see `in the mind's eye' that the area is tiny compared with the original square, and so the mathematical problem is answered by this simple thought –experiment..

10. A very famous thought experiment in science is due to Galileo. He proved that bodies of unequal weight must fall at the same rate. Before Galileo, people had assumed the opposite that heavier bodies fall faster. Let us suppose that that assumption were true, and imagine a falling heavy body catching up with a slower light one, and the two fusing. The slower body must slow the faster one down (cf. grabbing the saddle of a faster cyclist overtaking you) so the fused body now moves slower than the heavy body; but the fused body, since it's heavier than the heavy body must, by our starting assumption, move faster. These last two conclusions are contradictory, therefore the original assumption must be rejected (reductioad absurdum).

11. Another brilliant thought experiment due to Galileo: He argued from the law of equal heights (an (ideal) ball on a curved track will retrieve its original height) to a substantive conclusion refuting Aristotle's theory of motion. Lengthen one side of the track -- ball returns, on a longer path, to its original height. Now lengthen that side by laying it horizontal to infinity. Since the law of equal heights says that the ball must continue to roll until it regains its original height, we can conclude that the ball will continue forever in a straight line. This result destroys Aristotle's supposition that some external force is needed just to keep things moving.

12. Simon Stevin's thought experiment (1605) to show that the force needed to stop an object sliding down a plane is (as we should say) proportional to the sine of the incline. This seems to be a solid scientific result, but it is not obtained by scientific experimentation, in the traditional sense. Are such thought experiments scientifically acceptable. Do they deliver us scientific knowledge that is as secure as the knowledge that results from normal experimentation?