"The ideas of the ruling class are in every epoch the ruling ideas, i.e. the class which is the ruling material force of society, is at the same time its ruling intellectual force. The class which has the means of material production at its disposal, has control at the same time over the means of mental production, so that thereby, generally speaking, the ideas of those who lack the means of mental production are subject to it."

I know this quote is from The German Ideology, which I have not read yet because I haven't finished Capital yet. Does Marx explain what the means of mental production are somewhere in The German Ideology or anywhere else? If not, where can I find an explanation?