A principal aim of medical research is to provide understanding of medical issues. Medical explanation has two dimensions, biomedical and epidemiological. Biomedical research is grounded in a hierarchical explanatory model, namely a vertical biomedical reductionist, hierarchical model (pathogenesis), while the epidemiological approach is horizontal. The clinical facts are pivotal, because the normal/pathological conventional split hinges on them and grants an epistemic priority to the clinical situation. Reductionism might have to be weakened and replaced either by a supervenience story or emergentism. Finally, Darwinian explanation with the Panglossian fallacy has limited explanatory power in medicine. There are no laws in biology.