This, the firm control of the senses, is what is called
yoga. One must then be vigilant; for yoga is subject to growth and decay.
(Katha Upanishad, 2.3.11)
If the mind ever so slightly strays from the ideal and
becomes outgoing, then it goes down and down, just as a ball inadvertently
dropped on the staircase bounces down from one step to another.
(Vivekachudamani, 325)
…inadvertence is death itself, and mindfulness is
immortality. It is due to inadvertence that the asuras were subjugated by
defeat and death, and it is by virtue of vigilance that the gods are immortal. (Sanatsujatiya,
4-5)