Christopher wrote:

> What's the name of the essay?

It's [Lectures on \\(n\\)-categories and cohomology](http://math.ucr.edu/home/baez/cohomology.pdf). For understanding fibrations and such, you want the part starting in Section 1.2 where I say

> **Connected covering spaces of \\(B\\) are classified by subgroups \\(H \subseteq \pi_1(B) \\).**

up to the part in Section 1.5 where I say

> **Fibrations \\(E \to B\\) where \\(E\\) and \\(B\\) are \\(n\\)-groupoids are classified by weak \\((n + 1)\\)-functors \\(B \to n\textrm{Gpd}\\).**

Both these mottos, and many more, are fancied-up versions of the very basic one I forgot to say:

> **Subsets \\(E \subseteq B\\) of a set \\(B\\) are classified by functions \\(B \to \lbrace 0,1\rbrace \\).**

I then go on to flesh out the philosophy connecting this idea to cohomology. In the process I _do_ explain why truth values \\( \lbrace 0,1\rbrace\\) are \\(n\\)-groupoids with \\(n = -1\\)! That's in Section 2, "The power of negative thinking".