The homeopathic principle of similitude is not based on the principle of vaccination but on the more general principle of inversion of effects, a widespread medical phenomenon. Based on the systemic networks which play an important role in response to stress, this principle concerns the reorganization of regulation systems, through a coherent response to the medicine. This model is backed by a large number of published studies from our laboratory and others, by toxicological evidence such as the emerging fields of ‘hormesis’, of neuropharmacology, and of systems biology. The immune system is just one example of the possible application of this law of similitude, which constitutes the true ‘heart’ of the homeopathic idea, and again ethically justifies the attempt to apply it to experimental pharmacology.