With regard to the nature of disease, Galen and Mesmer were metaphysical monists. There is only one disease, and only one cure for it.

For Mesmer, all disease is caused by disturbance in the flow within the body of a universal and all-pervading fluid. Therefore, all cures do the same thing—they correct the flow of the vital fluid by means of magnets, mesmeric passes, and suggestion.

For Van Helmont, diseases have distinct etiologies and deserve different kinds of treatment.

We know this, interestingly, from how they proposed testing the efficacy of their respective treatments. Both called for randomly selecting patients by lots. But Mesmer wanted his sample to be a mixture of patients, while Van Helmont called for only patients with fevers (to test the efficacy of bloodletting).

Today we know that diseases are multiple. Biology is the most complicated of all human sciences—different diseases deserve their own specialists. But while biological systems are widely differentiated, biology rests on chemistry rests on physics, and physics has had great success reducing (predicating) everything from a handful of simple laws.

So how modular is life, really? The reduction of complexity in forces and matter to the tiny set of entities in the Standard Model is proof that, for human minds at least, wholes are best understood through the process of analysis–synthesis. So there really is only one disease—deviation of subatomic motion away from healthy biological process. It’s only when you zoom out and make things fuzzy that modularity comes into view. So Galen/Mesmer and Van Helmont are both correct.