²It has also been claimed that other types of cells in the brain could also participate in computations, in particular glial cells (Perea et al., 2009). However, AI systems only use a single kind of cells, mimicking neurons.— I do not attempt to define “information processing” in any rigorous way in this book. It is a very general concept with many meanings, and attempting to define it in a way that is both general and rigorous enough seems hopeless to me.