Please answer a question about the following article about Age of Enlightenment:

The Republic of Letters was the sum of a number of Enlightenment ideals: an egalitarian realm governed by knowledge that could act across political boundaries and rival state power. It was a forum that supported "free public examination of questions regarding religion or legislation". Immanuel Kant considered written communication essential to his conception of the public sphere; once everyone was a part of the "reading public", then society could be said to be enlightened. The people who participated in the Republic of Letters, such as Diderot and Voltaire, are frequently known today as important Enlightenment figures. Indeed, the men who wrote Diderot's Encyclopédie arguably formed a microcosm of the larger "republic".

What did Immanuel Kant consider essential to his conception of the public sphere?
written communication