 This physiological stratum of Bacon's natural philosophy was influenced by his semi-Paracelsian cosmology (on Paracelsus see Müller-Jahncke 1985, 67–88), which Graham Rees (Rees and Upton 1984, 20–1) has reconstructed from the extant parts of the Instauratio Magna. Detailed consideration therefore has to be given to Bacon's theory of the ‘quaternions’.