The argument between the biometrical school and the Mendelians is one of the most often-cited debates on the structure of evolutionary theory in the years immediately following Darwin's death in 1882. I argue that we can only make sense of it if we see the Batesonians as reacting against the philosophical content of the biometricians' work – particularly, the influence of Machian phenomenalism on Karl Pearson, and W.F.R. Weldon's views on causation.