I think it's worth pointing out that "Perceptions" also says it's trivially obvious that any network with a hidden layer could be used to approximate any curve

I think it's worth pointing out that "Perceptions" also says it's trivially obvious that any network with a hidden layer could be used to approximate any curve because then you're basically manually tweaking an arbitrarily complex function. This was significant at the time because no one had published a training algorithm for networks with hidden layers (e.g. backpropogation)