In the Synthesis itself and in the same chapter Sri Aurobindo also says this.
‘The intellect has been interested, the heart attracted, the will has strung itself to the effort, but the whole nature has not been taken captive by the Divine. It has only acquiesced in the interest, the attraction or the endeavour. There has been an experiment, perhaps even an eager experiment, but not a total self-giving to an imperative need of the soul or to an unforsakable ideal. Even such imperfect Yoga has not been wasted; for no upward effort is made in vain. Even if it fails in the present or arrives only at some preparatory stage or preliminary realisation, it has yet determined the soul’s future.’
In other words it is a journey of the Divine unfolding within us. It is therefore quite natural that there would be steps and stages and different degrees of engagement before the decisive turn happens. Everyone goes through this process, sometimes a long process of preparation in this life or previous ones. And even after the decisive turn, the journey has its own unique challenges for each one.
But apart from that the Yoga itself became simpler and easier with the coming of the Mother and taking up the charge of the Yoga completely from 24th November 1926. Here is what Sri Aurobindo writes over a decade later in his letters.
‘I cannot understand whether I am doing Yoga. Can it be said that I am doing your Purna Yoga?
Everyone who is turned to the Mother is doing my Yoga. It is a great mistake to suppose that one can “do” the Purna Yoga—i.e. carry out and fulfil all the sides of the Yoga by one’s own effort. No human being can do that. What one has to do is to put oneself in the Mother’s hands and open oneself to her by service, by bhakti, by aspiration; then the Mother by her light and force works in him so that the sadhana is done. It is a mistake also to have the ambition to be a big Purna Yogi or a supramental being and ask oneself how far have I got towards that. The right attitude is to be devoted and given to the Mother and to wish to be whatever she wants you to be. The rest is for the Mother to decide and do in you.
April 1929
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I offer myself at your feet. Accept me as your child and show me the divine path. Give me directions and inform me what will be the attitude in my sadhana.
Write to him that he can begin sadhana, if he feels truly the call. He need do nothing at first but sit in meditation for a short time every day and try to open himself to the Mother’s power, aspiring for the opening, for a true change of consciousness, for peace, purity and strength to go through the sadhana, for her protection against all difficulties and errors and for an always increasing devotion. Let him see first if he can thus successfully open himself.
2 November 1929
You can see for yourself how the Integral Yoga itself evolved for mankind from the days of the Synthesis to the days of the Mother taking charge of the aspirants.