Arguing that medicine must become a true experimental science, the work lays out principles for designing controlled experiments, forming and rigorously testing hypotheses, isolating variables, and seeking reproducible physiological laws; it defends the use of animal experimentation to reveal causal mechanisms, introduces the critical idea of the internal milieu and its constancy as the basis of health, and stresses that experimental findings should overturn preconceived notions and be translated to clinical practice with caution and rigor.