In this study, subjects were made to sleep with their eyelids open, thus allowing the visual cortex to receive visual stimuli. Though their eyes were open, and the perceptual input was accessible, the subjects could not see the stimuli and did not report dreaming of it. It is the brain that is having the internal experience, independent of perceptual input. This internalist view of consciousness leads Revonsuo to compare both dreaming and waking consciousness with a virtual reality simulation decoupled from or only indirectly informed by a brain's external environment. Philosophically, Revonsuo's claim that dreaming is a state of consciousness at all contradicts arguments propounded by philosophers Norman Malcolm and Daniel Dennett. Malcolm argues that, if a person is in any way conscious, "it logically follows that he is not sound asleep". Dennett suggested that we needed a well-confirmed, empirical theory of dreams before we could say whether dreams were like experiences or not. Revonsuo's threat simulation theory claims that much or all of dream experience is "specialized in the simulation of threatening events", for the evolutionary purpose of rehearsing fight or flight situations to better prepare for such instances in waking life (similar to a "fire drill"). According to Revonsuo, empiricial research supports this theory by showing the recurrence of threatening situations in dreaming: of all of the emotions experienced in dreaming, "fear [is] the most common and anger the next most common". An otherwise supportive 2009 review of threat simulation theory stated that "The main weakness of the theory is that there is no direct evidence of the effect of dream rehearsal (or the lack of it) on performance or on survival rates across generations of ancestral humans", and also notes it is not clear why some threat simulations end without a reaction from the dreamer. The review finds that "Overall, the available new evidence and the new direct tests of the predictions of (threat simulation theory) yield strong support for the theory. A mass of evidence indicates that threat simulation is a function of dreaming, an evolved psychological adaptation selected for during the evolutionary history of our species. On current evidence, the strengths of the theory seem to outweigh its weaknesses." According to a 2017 study in "Sleep", an analysis of the statistical content of intelligible sleep-talking found that 24 percent contained negative content, 22 percent had "nasty" language, about 10 percent contained a variation of the word "no", and 10 percent contained profanity. 2.5 percent of the intelligible words were a variation of the word "fuck", which comprised only 0.003 percent of spoken words when awake.