128 Will Robots Inherit the Earth? The brain, like other finite things, must reach some limits to what it can learn. We do not know what those limits are; per- haps our brains could keep learning for several more centuries. Ultimately, though, we will need to increase their capacity. The more we learn about our brains, the more ways we will find to improve them. Each brain has hundreds of specialized regions. We know only a little about what each one does – but as soon as we find out how any one part works, researchers will try to devise ways to extend that organ’s capacity. They will also conceive of entirely new abilities that biology has never provided. As these inventions accumulate, we will try to connect them to our brains – perhaps through millions of microscopic electrodes inserted into the great nerve-bundle called the corpus callosum, the largest data-bus in the brain. With further advances, no part of the brain will be out of bounds for attaching new accessories. In the end, we will find ways to replace every part of the body and brain – and thus repair all the defects and flaws that make our lives so brief. REPLACING THE BRAIN Almost all the knowledge that we learn is embodied in vari- ous  networks  inside  our  brains.  These  networks  consist  of huge numbers of tiny nerve cells, and even larger numbers of smaller structures called synapses, which control how signals jump  from  one  nerve  cell  to  another.  To  make  a  replace- ment of your brain, we would need to know something about how each of your synapses relates to the two cells it bridges. We would also have to know how each of those structures responds to the various electric fields, hormones, neurotrans- mitters, nutrients and other chemicals that are active in its neighborhood. Your brain contains trillions of synapses, so this is no small requirement.