“Words, words, words, I’m so sick of words!”

Many scientists seem to agree with a favorite adage of the best film directors: “show, don’t tell”. A look at the latest articles in Nature will often reveal that half of the available space is devoted to pictures and diagrams rather than text. Supplemental materials may even consist exclusively or almost exclusively of diagrams.

While historians of science have long paid attention to visual representations, philosophers have by and large ignored them. But this is slowly beginning to change. In the last decade or so there have been recurrent bubbles of philosophical interest in diagrams, including a project directed by William Bechtel and Adele Abrahamsen at UCSD under the lovely acronym Worgods (Working Group on Diagrams in Science). While I was at the Pittsburgh Center with the two of them, I quickly recognized not only how important diagrams are in scientific practice, but also that they had figured prominently in much of my previous research. I had just never stopped to consider them as objects of inquiry in their own right.

Take this diagram as an example:

Spot the difference figures
A figure from Fire et al. in Nature, 1998, volume 391, p. 808.
The figure appeared in a 1998 paper in Nature by Fire et al. It shows fluorescence micrographs of green fluorescent protein (GFP) in C. elegans. In a and b, GFP is expressed in a larva and in an adult, respectively. In d and e, the expression is suppressed. In g and h, the expression is suppressed in the nucleus, but not in mitochondria. What is the point of figures like this one?

In a forthcoming paper, I give a pretty straightforward answer. Many of the diagrams in your routine scientific publication depict what I call “causal contrasts”. They show what happens to a particular outcome variable if a specific intervention is performed, comparing this to a control in which the intervention is not performed. In the diagram above, the point is to show that an intervention with double-stranded RNA can suppress the expression of sequence-homologous genes (compare a and d, b and e). What is more, the diagram shows the specificity of this effect: if the double-stranded RNA is targeted only against the nuclear GFP gene, then the expression of mitochondrial GFP remains unaffected (compare a and g, b and h). For their demonstration of this extremely effective technique for gene suppression, the authors received the 2006 Nobel Prize in physiology or medicine.

I argue in the paper that many diagrams show causal contrasts, even though they differ significantly on the surface. Causal contrasts appear in many guises, some more obvious than others. They also appear in many scientific contexts, from the experimental to the observational to the purely theoretical.

Causal contrast diagrams are philosophically significant. They are a window into one of the key practices of scientific epistemology: causal inference. I suggest that this goes far in explaining why scientists, when reading a paper, turn to the diagrams first. A study’s key results can often be found there. Intriguingly, diagrams are often much more than merely a preferred representational tool for causal inferences. Diagrams themselves often constitute evidence: think of the ubiquitous photographs of electrophoresis gels in molecular biology, or the fluorescence micrograph shown above.

I call the paper “Spot the difference: Causal contrasts in scientific diagrams”. A preprint is available on the PhilSci archive, and the finished paper is about to come out in Studies in History and Philosophy of Biological and Biomedical Sciences.