Yoga sutra 1.35
viṣayavatī vā pravr̥tti-rutpannā manasaḥ sthiti nibandhinī
A peaceful mind can also result from intelligent activity in the domain of sensory objects to promote stability
We are, more often than not, salves to our sensory perceptions. We wake up, pour ourselves a cup of ambition, turn the TV on or play the radio, spend most of the day on a computer, etc. etc. Yoga takes us in the opposite direction; turning your senses inwards.
You cannot think two thoughts at the same time. In other words, there cannot be more than one activity in the mind at a time. If the mind is fixed on one such activity, it is pacified. If the mind is limited to a single intelligent activity for a very long time, a peaceful mind is achieved. This is the principle behind the technique Trataka, that is a meditation technique which involves focusing the eyes (and, in turn, the mind) through intent but relaxed gaze on a flame from a lit candle. Other practices include being out in nature and meditating on a beautiful landscape.
Damian Cadman-Jones is an authorised teacher of Prana Vashya Yoga™