"Come on now, we're going to go build a mirror factory first and put out nothing but mirrors for the next year and take a long look in them."
Granger suggests that building a mirror factory is the first thing that they do to rebuild a society because that was the biggest problem in Montag's society. People stopped looking at who they really were and just became whoever and whatever the society dictated. Firemen were firemen by virtue of their looks. Any men born fitting the physical description of a fireman were fireman, no questions asked. By creating mirrors a society can decide for themselves who they are and what they want to become. They can decide things for themselves rather than allowing the government to decide for them. Knowing who you are as a person is important to societies in general because it allows for diversity and it also makes it difficult for a government to gain total control.
Mirrors are a motif throughout Fahrenheit 451.For instance, at the beginning, Clarisse is described as a mirror. Also, when Montag really starts to examine his life and society as a whole, he begins by looking at himself in the mirror, noticing his burnt-skin, but he no longer takes pride in it. Therefore, it is only fitting that the novel ends with a mention of mirrors. It solidifies the concept of the need for self-reflection. If the new society only produced mirrors, everyone would have to look at themselves from all angles and really judge who they are and what they do. The current society does not do that at all.
Granger is saying that the society needs to reevaluate what they have done with their lives and their worlds by looking in a mirror.
The mirror factory must be understood as a metaphor. To be able to change the attitude of society, they need a reflection of what they have done. In contrast to the parlor walls, mirrors are made for that. They reflect the person who is looking inside as they are. The parlor walls of Fahrenheit 451 just showed the humans what they wanted to see without giving them the possibility to see the real world.By entertaining them all tht time, they make them losing the reality of life.
The unconcealed look of a mirror would be necessary for the society of Fahrenheit 451 to have a chance to change something after the war.
With self-reflection, they should be able to avoid the repetition of former society catastrophes, a system which is described by Karl Marx in his state-theory.
Granger wants to try to change this circle of history by showing the humans their mistakes, like a mirror shows a human itself.