The paper aims to show that scepticism concerning the status of first-person reports of mental states and their use as evidence in scientific cognitive research is unfounded. Rather, principled arguments suggest that the conditions for the intersubjectivity of cognition and description of publicly observable things apply equally for our cognition and description of our mental or internal states. It is argued that on these conditions relies the possibility of developing well-defined scientific criteria for distinguishing between first-person and third-person cognition and description. The paper concludes by outlining the consequences for cognitive research and for functional theories of mind.