I've managed to confuse myself over this detail:
Obviously: $n^2 \notin \Theta(n)$
Now if we take the $\log$ of both sides, we get:
$$\log(n^2) \leq \log(cn)$$
$$2\log(n) \leq \log(c) + \log(n)$$
Suddenly this equation looks true... that you can find a $c$ to satisfy the $\Theta$ definition. Obviously, I know this is not the case. Question is, when can you take the log of both sides to prove a O/Omega/Theta definition? What about for $2^n \in \Theta(n!)$?