Dreams can link to actual sensations, such as the incorporation of environmental sounds into dreams such as hearing a phone ringing in a dream while it is ringing in reality, or dreaming of urination while wetting the bed. Except in the case of lucid dreaming, people dream without being aware that they are doing so. Some philosophers have concluded that what we think as the "real world" could be or is an illusion (an idea known as the skeptical hypothesis about ontology). The first recorded mention of the idea was by Zhuangzi, and was also discussed in Hinduism; Buddhism makes extensive use of the argument in its writings. It was formally introduced to western philosophy by Descartes in the 17th century in his Meditations on First Philosophy.

Main article: Dream argument