By Swami Chidananda
The Upanishads supply a litmus test for us to know if our life has the stamp of Yoga. Neither theoretical knowledge nor being able to do difficult postures (or breathing exercises) would make us yogis. It is rather that natural alertness in which all our sense organs behave properly, avoiding excess anywhere.
They call that YOGA when there is steady control over the senses. Then one is alert, no more prone to error. (Otherwise generally) yoga is marked by rise and fall.
tām yogam-iti manyante
sthirām-indriya-dhāranām
apramattas-tadā bhavati
yogo hi prabhavāpyayau
Katha Upanishad 2.3.11