Ewan McGregor probably knew he was hallucinating when he saw a dead baby crawl across the ceiling in Trainspotting, but are you sure you aren't dreaming right now? According to philosophy professor and metaphysics researcher Jan Westerhoff, there's a relatively high probability that you are. In his book Reality: A Very Short Introduction, Westerhoff argues that realistically, the odds of you dreaming at this very instant are about ten percent.
Brain Pickings has an explanatory excerpt:
Lets do a quick calculation. We optimistically assume that you get eight hours of sleep a night, which leaves sixteen hours during which you are awake. Sleep researchers have found out that there is a strong correlation between dreaming and being in so-called REM (rapid eye movement) sleep. REM sleep is characterized by rapid movement of the eyeballs; the brain is highly active, its electric activity resembles that of a waking brain, but the sleeper is more difficult to wake than during slow-wave or non-REM sleep. We know that between 20% and 25% of our sleep is REM sleep. Taking the lower value and assuming that you always and only dream during REM sleep, this gives us 1.6 hours of dreaming ever[y] night. As there are therefore 1.6 hours of dream consciousness for every 16 hours of waking consciousness, this means that your chance of dreaming at any given moment is 1 in 10.
Compare that with the odds of winning the lottery (one in 14 million) or getting struck by lightning (one in 1 million) and the line separating "reality" from an arachnoid baby may start to blur. Maybe.