The Cerebral Cortex is the surface region of the brain that is most strongly linked to intelligence. A human’s cerebral cortex, if flattened, would cover four pages of typing paper; a chimpanzee’s would cover only one page; and a rat’s would cover a postage stamp.
There are three levels of self to consider: the proto, the core, and the autobiographical. The first two are shared with many, many other species, and they are really coming out largely of the brain stem and whatever there is of cortex in those species.
Speech , on account of the whole preceding life of the adult, is connected up with all the internal and external stimuli which can reach the cortex, signaling all of them and replacing all of them, and therefore it can call forth all those reactions of the organism which are normally determined by the actual stimuli themselves. We can, therefore, regard ‘suggestion’ as the most simple form of a typical reflex in man.