The idea behind backpropagation
	• We don’t know what the hidden units ought to do, but we can compute how fast the error changes as we change a hidden activity. –  Instead of using desired activities to train the hidden units, use error derivatives w.r.t. hidden activities. – Each hidden activity can affect many output units and can therefore have many separate effects on the error. These effects must be combined. – We can compute error derivatives for all the hidden units efficiently. – Once we have the error derivatives for the hidden activities, its easy to get the error derivatives for the weights going into a hidden unit.