What a middle unit is for

Those four new units have a different job from anything in the last chapter.

The units on the left report facts supplied by the wine: cabernet, viognier, plum, and apricot. The unit on the right will report the machine's final answer, a price.

The four in the middle are neither the supplied facts nor the final answer. Each one reports a conclusion formed inside the machine:

This bottle is a plummy cabernet.

That is not one of the four facts the bottle supplied. It is a new signal assembled from two of them.

Which is why it deserves the word that gets used for it. These units are hidden.

Hidden does not mean secret or deliberately concealed. It means these units sit inside the machine, between its inputs and its answer. A bottle does not arrive with their correct values written on it, and the merchant never tells the machine what they should report.

That last part is going to matter enormously in a few slides.

The useful way to think of a hidden unit is as a question the machine asks itself on the way to an answer.

The four here ask whether the bottle is one particular type of wine. We chose those questions ourselves when we set the connections on the last slide. This machine has not learned them yet.

For now, that choice lets us see what a middle row makes possible. Later, the machine will have to discover useful questions from the prices alone.

A row of hidden units like this is called a hidden layer.