Human consciousness and the very existence and concept of the mind or soul seem to pose the metaphysical touchstone for philosophers and physicist alike. Epiphenomenalism claims that mental states are caused by physical states but are themselves causally inert. Thus, by virtue of the epiphenomenal nature of mental states the very existence of mental states can be questioned (by a materialistic stance). If they don’t exert any causal influence, then they’ll likely resist any physicalist explanation. And if mental states are causally inert do we have any reason to believe they exist? This essay aims to answer the question how we can believe that mental states (qualia and propositional attitudes) exist by first analysing the nature of mental states, secondly by challenging physicalist epiphenomenalism and finally by stating that the epistemic ability to capture reality is one aspect but the ontological possibility of a priori knowledge and a posteriori knowledge by qualia is another.