 Simple module of a finite-dimensional commutative complex algebra

For a finite-dimensional commutative complex algebra, a simple module is a cyclic quotient of the algebra and hence finite-dimensional. Every action map is a module endomorphism, since the algebra is commutative. The complex Schur's lemma makes each action scalar, so every vector subspace is a submodule; simplicity forces dimension one. The finite-dimensional hypothesis cannot simply be deleted: $\mathbb{C}(t)$ acting on itself is a simple module of infinite complex dimension.