"Beware, the life of this world is nothing but a pastime, a momentary delight. It is but empty bragging, a pursuit of wealth and children. It is like the vegetation that flourishes after rain and is a source of delight to the farmer, but soon turns yellow and withers away, crumbling into worthless waste. But in the life to come an eternal contentment or a grievous suffering awaits you. The life of this world is only an illusion."
Qur'an, Al-Hadid, Surah 57:20

It fascinates me that Islam, Buddhism, Hinduism, Taoism and to some extent Christianity all directly or indirectly refer to the life of the world as illusion. While I can grasp the concepts of gross, subtle and causal levels of awareness/consciousness when related to the waking state, the dream state, and deep dreamless sleep, and while I understand (or think I do) the concepts of modern physics which hold that all reality is merely a collection of vibrating strings, just how that applies to "reality" that I can touch, taste, see and smell eludes me. Anyone out there who can shed some light on the meaning of "illusion" as used in these various spiritual tracts?