Here, we're going to describe whatever I've picked up of sheaves in the past
couple of weeks. I'm trying to understand the relationship between sheaves,
topoi, geometry, and logic. I currently see how topoi allows us to model logic,
and how sheaves allow us to model geometry, but I see nothing about the
relationship! I'm hoping that writing this down will allow me to gain some
perspective on this.

Let's consider two sets $P, A$, $P \subseteq A$. Now, given a function
$f: A \rightarrow X$, we can restrict this function to $A_P: P \rightarrow X$.
So, we get to invert the direction :

$(P \subseteq A) \iff (f: A \rightarrow X) \rightarrow (f_P: P \rightarrow X)$

We should now try to discover some sort of structure to this "reversal"
business. Perhaps we will discover a contravariant functor! (Spoiler: we will).