  Now suppose instead that we say that if a conscious mental event is introspectively indiscriminible from a genuine perception it has the same phenomenal properties as a genuine perception. According to Martin this would be to embrace a substantive epistemic principle. For in adopting this stance one would be committed to the epistemic claim that in the case of any conscious mental event that lacks the phenomenal properties of a genuine perception, the fact that it lacks the relevant phenomenal properties will be something that is knowable through introspection. According to such a view, it is not just that the phenomenal properties that determine an event as an experience are held to be introspectively detectible and recognisible as such when present. It must also be the case that the absence of such properties, when they are absent, is equally detectible through introspection.