Such ideas greatly influenced mediaeval religious thought and are visible in the Renaissance medicine of Paracelsus and Servetus. In the romantic era, alongside the discovery of electromagnetism and the nervous system, there came a new interest in the spirit world. Franz Anton Mesmer spoke of the stars, animal magnetism and magnetic fluids. In 1801, the English occultist Francis Barrett wrote of a herb's "excellent astral and magnetic powers" - for herbalists had categorised herbs according to their supposed correspondence with the seven planetary influences.