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".