We prove there is no sentence in the monadic second-order language
M
S
0
MS_{0}
that characterises when a matroid is representable over at least one field, and no sentence that characterises when a matroid is
K
\mathbb {K}
-representable, for any infinite field
K
\mathbb {K}
. By way of contrast, because Rota’s Conjecture is true, there is a sentence that characterises
F
\mathbb {F}
-representable matroids, for any finite field
F
\mathbb {F}
.