  Thomassen (1994) proved that every planar graph is 5-choosable. This result
was generalised by {\v{S}}krekovski (1998) and He et al. (2008), who proved
that every $K_5$-minor-free graph is 5-choosable. Both proofs rely on the
characterisation of $K_5$-minor-free graphs due to Wagner (1937). This paper
proves the same result without using Wagner's structure theorem or even planar
embeddings. Given that there is no structure theorem for graphs with no
$K_6$-minor, we argue that this proof suggests a possible approach for
attacking the Hadwiger Conjecture.
