In mental spaces theory, the 'If A then B' construction serves as a space builder to set up two successive spaces: the foundation space and the expansion space. The foundation space is a hypothetical mental space set up by the space builder if. The expansion space is set up by the space builder then. While the foundation space is hypothetical relative to the base space, whatever holds in the foundation space is 'fact' relative to the expansion space and the structure in the expansion space is entailed by it. For instance, in the utterance, If I won the lottery then I would buy a Rolls-Royce, the if clause builds a space in which I win the lottery. The then clause builds a space in which, given this 'fact', I buy a Rolls-Royce.