 By contrast, it has been discovered empirically that water is H2O. The second constraint binds what is true at other possible worlds to facts about how things actually are. As Chalmers explains, that captures a kind of subjunctive possibility. Since chemical composition is arguably essential to substances, such as water, it is not secondarily conceivable that water is not H2O. As Chalmers also puts it, secondary conceivability amounts to coherently conceiving the situation described as counterfactual, namely as a way the world might have been, given how our world is. Thus: