Let us contemplate on the difference between Atma (the soul or the Self) and the mind. The soul is unchanging, but the mind keeps on changing from time to time. This is why our mind has been linked to the moon (referring to the different phases of the waxing and waning moon). It is said, 'Chandrama manaso-jayate' (from the Purusha Sukta in Rig Veda; meaning: From His Cosmic Mind was born the Moon). There is a deep connection between the moon and the mind. Just as the moon waxes and wanes over the days, in the same way our mind also experiences ups and downs.
Sometimes it expands, sometimes it contracts. Sometimes it is happy without any reason, and sometimes it is miserable without any reason. This is why people through the ages have said, 'One who conquers the mind conquers the entire World'. The mind alone is the reason behind bondage and liberation. Whether we experience some bondage or attachment, or whether we feel free, the reason for this is the mind. So we have to train the mind again and again. This is the essence of knowledge. As we go on practising this regularly, we gradually attain the siddhi (extraordinary ability) to tame the mind -- just as a person, after repeatedly going through problems in life gradually gains the ability to smile through them.
Then a stage comes where he does not feel any misery in spite of a problem. As we go through the experiences of life (whether pleasant or unpleasant), a stage comes when this realization dawns within us that 'Everything is nothing'. That is when we begin to get established in the Self. To be established in the Self means that regardless of the person, object or situation that comes before us, we do not lose our smile and equanimity even for a moment.
Even if we do lose it, it is momentary and we instantly gain it back. Then we are strongly and firmly established in the Self. Then it is said that one has become a Siddha (a perfected one), that one has truly become established in the Self.