We have noted already (Chapter 2, section B) that mental processes correspond to or are identical with physiological processes taking place within the brain (or also the nerves and sense organs). It seems most probable that the phylogenetic mental development from lower to higher animals and then on to man ran parallel with that of the nerves, sense organs and brain. But as no reliable criteria for mental evolution of animals exist, we are obliged to rely on assumptions based on analogy. What we can deduce from these, however, in some aspects amounts to virtual certainty [181, 195, 196].