"One of the key concepts in Buddhism. Things in the phenomenal world are not real or substantial， as ordinary people regard them to be. They are transient， momentary， indefinite， insubstantial， and subject to constant alteration. In reality they are like phantoms or hallucinations."
"Phenomenal 'existence，' as commonly perceived by the senses， is illusory; it is not real inasmuch as， though it exists， its existence is not permanent or absolute. Nothing belonging to it has an enduring entity or 'nature' of its own; everything is dependent upon a combination of fluctuating conditions and factors for its seeming 'existence' at any given moment.' Thus we have the expression， 'illusory but not non-existent.'"