Omissions figure prominently in causal reasoning from diagnosis to ascriptions of negligence. One philosophical proposal posits that omissions are accompanied by a contrasting alternative that describes a case of orthodox (nonomissive) causation (Schaffer, 2005; Bernstein, 2014). A psychological hypothesis can be drawn from this contrast view of omissions: by default, humans should interpret omissive causations as representing at least two possibilities, i.e., a possibility representing the omission and a possibility representing a contrast. The theory of mental models supposes that reasoners construct only one possibility (the omission) by default, and that they consider separate alternative possibilities in sequential order. Two experiments test the contrast hypothesis against the model theory, and find evidence in favor of the model-theoretic account.