Another seminal passage from Freud (Beyond the Pleasure Principle)

"Let us picture a living organism in its most simplified possible form as an undifferentiated vesicle of a substance that is susceptible to stimulation. Then the surface turned towards the external world will from its very situation be differentiated and will serve as an organ for receiving stimuli.... It would be easy to suppose, then, that as a result of the ceaseless impact of external stimuli on the surface of the vesicle, its substance to a certain depth may have become permanently modified, so that excitatory processes run a different course in it from what they run in the deeper layers. A crust would thus be formed which would at last have been so thoroughly 'baked through' by stimulation that it would present the most favourable possible conditions for the reception of stimuli and become incapable of any further modification...."

"But we have more to say of the living vesicle with its receptive cortical layer. This little fragment of living substance is suspended in the middle of an external world charged with the most powerful energies; and it would be killed by the stimulation emanating from these if it were not provided with a protective shield against stimuli. It acquires the shield in this way: its outermost surface ceases to have the structure proper to living matter, becomes to some degree inorganic and thenceforward functions as a special envelope or membrane resistant to stimuli. In consequence, the energies of the external world are able to pass into the next underlying layers, which have remained living, with only a fragment of their original intensity; and these layers can devote themselves, behind the protective shield, to the reception of the amounts of stimulus which have been allowed through it. BY ITS DEATH, the outer layer has saved all the deeper ones from a similar fate- unless, that is to say, stimuli reach it which are so strong that they break through the protective shield. PROTECTION AGAINST stimuli is an almost more important function for the living organism than RECEPTION OF stimuli...." (p. 20, 21)