Shepherd vol 2 ebook: Chapter 9: Experimentum Crucis

Chapter 9: Experimentum Crucis

In this chapter, I shall be focusing on the scholarly phrase: Experimentum Crucis, which Mary Shepherd uses more than once across both her 1824¹ and 1827² treatises. It is a phrase that was made famous by Isaac Newton, although it was originally used by Robert Hooke³. Nevertheless, the concept of Experimentum Crucis was already present in Francis Bacon's philosophy of science, he just used the phrase Instantia Crucis instead⁴. As Sinico points out: “There is a long and rich debate in philosophy of science about experimentum crucis, which has concerned important philosophers”⁵. I shall be examining the topic of Experimentum Crucis, which spans the fields of pure science, logic, methodology and the philosophy of science, across three chapters in this volume (this one and the next two chapters). 

The scope of this chapter will focus on the question: What does Experimentum Crucis mean and does it provide us with scientific consensus for a materialistic, experimental understanding of the world? In the following chapter, I shall apply what we've learnt about the term and its methodological approach to one of Shepherd's texts and her philosophical and religious stances. 

Experimentum Crucis is a term that spans both the fields of logic and science and it is sometimes known by its English equivalent phrase: Crucial Experiment, which gives us a better sense of the meaning. It is an experiment that is crucial because it tries to find out which one, out of the several competing theories or hypotheses, is the most true, and which are the false ones. Or in other words, a more technical yet standard definition is that: “within Experimental Phenomenology, it is possible to obtain the falsification of one of two rival perceptual theories through experimenta crucis”⁶. Ideally, the scientist conducting the test or experiment will hope that the theory or hypothesis that is proved to be true then gains general consensus from the scientific community and remains true for all time. However, in practice, all scientific theories are based on provisional beliefs so they are only true as far as we can ascertain at that time and they are always potentially revisable beliefs and theories in the future, should they later be justifiably disproved. 

Moreover, consensus in the scientific community is not easy to obtain, even if you think that you have proven that your theory or hypothesis is true and that the other competing theories are false or less comprehensive explanations. This situation happened to the natural philosopher and scientist, Isaac Newton. He structured his Crucial Experiment such that only one of two theories could be proved right. Newton hoped that through an experiment he demonstrated with prisms, he could disprove his opponent’s Wave Theory (that light is composed of waves) and thereby prove his Particle Theory, as he irrefutably showed his hypothesis to be true, that light is composed of particles. Nevertheless, his peers were not convinced and doubted his findings, they could not replicate his experiment and results and continued to argue against his theory. Robert Hooke headed the counterargument against Newton, and many others joined him, including Kristen Huygens, the famous 17th century astronomer, physicist and natural philosopher who is credited with thinking up the Wave Theory of light in the first place. It is interesting to note that Huygens purchased and used Spinoza's high quality lens, despite mostly making his own lens for himself, and he conversed with Spinoza about optics and so on, which shows how Spinoza was an expert in science too, not just in philosophy. Huygens, Hooke and others continued to argue, undeterred by Newton's Experimentum Crucis, that Newton had not managed to debunk the Wave Theory of light. Hence, we can see that certainty is not common in science and consensus is rare. Not surprisingly, given that explanations are usually very complex and multilayered, with different factors at play, which may or may not be known at the time of experimentation. 

Nevertheless, Experimentum Crucis as a methodology has lasted the test of time and is still used today. As Achinstein states: 

“scientists frequently regard certain experiments as crucial in the sense that the experimental result helps make one theory among a set of competitors very probable and the others very improbable, given what is currently known.”⁷

Although Experimentum Crucis is mostly used as a method for practical science experiments, more than it is used for rational argumentation, Shepherd nevertheless invokes it in a few passages of her 1824⁸ and 1827⁹ treatises.

I suggest that it is more common for rational arguments to run along the logic lines of the format of the disjunct: either A or B. Not B therefore A. Here, B can be shown to not be the case, either through very simple empirical observation, or through the light of pure reasoning. For instance, I only have to step outside to test if it is raining or not, to show that the statement A: it is not raining, is true, and the statement B: it is raining is false and then formalise that into the conclusion not B therefore A. Newton had utilised this either/or framework too, since the only two plausible hypotheses at the time were the wave and particle theories, so refuting one would automatically seem to affirm the other. 

However, I suggest that an Experimentum Crucis adds more complex factors into the test, such as the role of the quality and clarity of a prism, how many prisms are used, what angle the light experiment is conducted, and so on. So we tend to gain a far more detailed understanding of the issues and the world around us when testing and disproving ideas in this Crucial Experiment manner. Nevertheless, no practical, experimental method is completely watertight. One has to be aware that there can always be a fly in the ointment that we did not accommodate, which ruins the experiment and provides us with false positives or negatives and inaccurate conclusions. Another area in which an Experimentum Crucis is rather tricky to perform is in the field of Social Psychology, although this is also arguably a general scientific issue, as can be seen when Forsyth maintains: 

“The difficulties in performing a crucial experiment in social psychology are addressed taking philosophical and epistemological issues into account. While previous discussions have proposed that present social psychological theories and methods prevent such tests, closer analysis indicates that this inability is not unique to social psychology, but is an aspect of any scientific inquiry.”¹⁰

Hence, although the Experimentum Crucis methodology is of great value in both science and philosophy, one cannot not assume it will provide an irrefutable conclusion or lead to academic or scientific consensus. Nevertheless, it is an important concept to grasp when reading Shepherd’s treatises, in order to understand not just her logical and philosophical approach but also her appreciation and depth of knowledge of scientific methodology and how it informs her philosophical argumentation and views. 

References:

¹Shepherd, Mary. An Essay upon the Relation of Cause and Effect : Controverting the Doctrine of Mr. Hume, Concerning the Nature of That Relation, with Observations upon the Opinions of Dr. Brown and Mr. Lawrence Connected with the Same Subject

(London, United Kingdom: Printed for T. Hookham, 1824), 

http://archive.org/details/essayuponrelatio00shepiala 

²Shepherd, Mary. Essays on the Perception of an External Universe and Other Subjects Connected with the Doctrine of Causation, 1st edition 

(Piccadilly, London, United Kingdom: J. Hatchard and son, 1827), 

http://archive.org/details/essaysonpercepti00shep. 

³Lohne, J.A., ‘Experimentum Crucis’, Notes and Records of the Royal Society of London 23, no. 2 (1968): 169–99, https://www.jstor.org/stable/530985 p169

⁴Ibid

⁵Sinico, Michele. ‘Why Experimentum Crucis Is Possible in Psychology of Perception’, Gestalt Theory 40, no. 1 (1 April 2018): 45–57, doi:10.2478/gth-2018-0003. p45

⁶Ibid 

⁷Achinstein, Peter. ‘Crucial Experiments’, in Routledge Encyclopedia of Philosophy, 1st ed. (London: Routledge, 2016), doi:10.4324/9780415249126-Q021-1

⁸Shepherd, An Essay upon the Relation of Cause and Effect 

⁹Shepherd, Essays on the Perception of an External Universe 

¹⁰Forsyth, Donelson R. ‘Crucial Experiments and Social Psychological Inquiry’. 

Personality and Social Psychology Bulletin 2, no. 4 (October 1976): 454–59. 

doi:10.1177/014616727600200420.