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How Many Bytes in Human Memory? by Ralph C. Merkle (appeared in Foresight Update No. 4, 1988) (merkle.pa@xerox.com) Today it is commonplace to compare the human brain to a computer, and the human mind to a program running on that computer. Once seen as just a poetic metaphore, this viewpoint is now supported by most philosophers of human consciousness and most researchers in artificial intelligence. If we take this view literally, then just as we can ask how many megabytes of RAM a PC has we should be able to ask how many megabytes (or gigabytes, or terabytes, or whatever) of memory the human brain has. Several approximations to this number have already appeared in the literature based on 'hardware' considerations (though in the case of the human brain perhaps the term 'wetware' is more appropriate). One estimate of 10**20 bits is actually an early estimate (by Von Neumann in 'The Computer and the Brain') of all the neural impulses conducted by the brain during a lifetime. This number is almost certainly larger than the true answer. Another method is to estimate the total number of synapses, and then presume that each synapse can hold a few bits. Estimates of the number of synapses have been made in the range from 10**13 to 10**15 -- with corresponding estimates of memory capacity. A fundamental problem with these approaches is that they rely on rather poor estimates of the raw hardware in the system. The brain is highly redundant and not well understood: the mere fact that a great mass of synapses exists does not imply that they are in fact contributing to the memory capacity. This makes the work of Thomas K. Landauer very interesting for he has entirely avoided this hardware guessing game by measuring the actual functional capacity of human memory directly ('How Much Do People Remember? Some Estimates of the Quantity of Learned Information in Long-term Memory' in Cognitive Science 10, 477- 493, 1986). Landauer works at Bell Communications Research -- closely affiliated with Bell Labs where the modern study of information theory was begun by C. E. Shannon to analyze the information carrying capacity of telephone lines (a subject of great interest to a telephone company). Landauer naturally used these tools by viewing human memory as a novel 'telephone line' that carries information from the past to the future. The capacity of this