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You are, as this book tries to make clear, your memories. That is, your personal identity is so intricately based on the sum total of your memories that it makes it impossible to have any idea of what a "self" may mean without resorting to understand how memory works. Since we take the memories we have to be the basis of our identity, it can be very hard to imagine that this memory has some serious limitations and ways that it can deceive us. A scientific study of memory is about a century and a half old, and over time we have managed to understand quite a bit about the inner working of human memory. The two main types of memory, short term and long term, are familiar to us from everyday life, but what is not too familiar is how short term memories get converted to the long term ones. This book gives an excellent account of this process, as well as how stable long term memories can be.
The book discusses the neurological basis of memory. All our memories are (at least for the foreseeable future) stored in our brains, and different parts of brain have a different function when it comes to the storage and retrieval of memories. A demage to any of those brain centers can have very serious and debilitating consequences for our normal cognitive functions.
A chapter of the book is dedicated to memory impairments, as well as to some reliable techniques for boosting one's memory. It also explains that there is an upper limit to how much we can remember. And that's a good thing - those few unfortunate individuals who could remember everything (mnemonists) ended up cluttering their minds with absolutely useless information, and normal human activities that we take for granted became impossible for them. It turns out, that we are not just what we remember: we are also what we forget. And that's worth keeping in mind.