"I do not think it is too far-fetched," Freud wrote, "to compare the celluloid and waxed paper cover with the system of Pcpt.-Cs. [Perception -Consciousness] and its protective shield, the wax slab with the unconscious behind them, and the appearance and disappearance of the writing with the flickering-up and passing-away of consciousness in the process of perception."
For Freud, this was new, as far as technology metaphors go. The other two major technologies he examines in his essay -- paper and slate -- he scrutinizes not so much as a metaphor for the mind, but in their capacity as memory aids or, "mnemic apparatus," as Freud calls them.
"Measured by this standard," Freud wrote, "devices to aid our memory seem particularly imperfect, since our mental apparatus accomplishes precisely what they cannot."
The first, paper and pen, preserves a thought -- a "permanent memory-trace" -- but it is finite and "the receptive capacity of the writing surface is soon exhausted." You then need more paper and more paper, a system for tracking it all, and you may soon lost track of these recorded memories, and forget they exist altogether, thereby negating the value of having recorded it in the first place. A slate, on the other hand, can be used over and over but nothing lasts very long. "Thus," Freud concluded, "an unlimited receptive capacity and a retention of permanent traces seem to be mutually exclusive properties in the apparatus which we use as substitutes for our memory: either the receptive surface must be renewed or the note must be destroyed."
Compare these technologies -- the slate and the pad of paper -- with the other "auxiliary apparatus" humans have designed to improve their sensory abilities: spectacles to improve vision or ear-trumpets for hearing. "Measured by this standard," Freud noted, "devices to aid our memory seem particularly imperfect, since our mental apparatus accomplishes precisely what they cannot: it has an unlimited receptive capacity for new perceptions and nevertheless lays down permanent -- even though not unalterable -- memory-traces of them."
This was why the Mystic Pad so intrigued Freud: It was a more precise analog of the mind's abilities. Like the Mystic Pad, "the perceptive apparatus of our mind consists of two layers," he observed, "of an external protective shield against stimuli whose task it is to diminish the strength of excitations coming in, and of a surface behind it which receives the stimuli." Spectacles, ear-trumpets, and the Mystic Pad all measured up to the human standard: By being like a human's abilities, they were well-suited to extending those abilities.
In that sense, computers are the perfect device for assisting our memories. They manage to solve Freud's conundrum: They have both an "unlimited receptive capacity for new perceptions" (more or less), and the ability to preserve "memory-traces" of them.