The external man is the outer human personality which is formed partly by the force of heredity and partly by the strong impressions and conditioning/imprints received from the early environment. These imprints are often further strengthened by habits reinforced by society. It has a stronghold upon our manners, the way we behave etc. It is this that comes as a front in our everyday interactions with people. Very often we try to gauge a person or the change in him through these observable elements. Sri Aurobindo is however reminding us not to judge what is going on within by the external man. Of course, this too must and does change but it is the last element.
Implicit is that there is behind the outer man, an inner being and an inmost being. Unlike in ordinary life where the stress is on modifying outer behaviour to suit our vested interests, yoga insists first on an inner change, a change of consciousness that then spreads authentically from within outwards. The outer man is the cover of the book of our life. The contents are the inner being while the intrinsic sense of the story, its central drift and purpose is the inmost being or the soul in man. It is this core aspiration of man, his central faith and will-to-be that is the real me. The outer also must change as a dress must indicate the true personality of the wearer, yet the reverse is not necessarily true. That is what Sri Aurobindo is revealing to us in the letter of April 24, 1934, as below.
Question: You have said that the aim of our Yoga is to rise beyond Nirvana, but even in the Ashram there are extremely few who have reached or have tried to reach even up to the Nirvana level. Even to reach Nirvana one has to give up desire, duality and ego and establish a certain amount of equanimity and peace. Could it be said that a sufficient number of sadhaks in the Ashram have succeeded in doing so? At least everybody must be making some effort to do this. Why then are they not successful? Is it that after some time they forget the aim and live here as in ordinary life?
Sri Aurobindo: I suppose if the Nirvana aim had been put before them, more would have been fit for it, for the Nirvana aim is easier than the one we have put before us – and they would not have found it so difficult to reach the standard. The sadhaks here are of all kinds and in all stages. But the real difficulty even for those who have progressed is with the external man. Even among those who follow the old ideal, the external man of the sadhak remains almost the same even after they have attained to something. The inner being gets free, the outer follows still its fixed nature. Our Yoga can succeed only if the external man too changes, but that is the most difficult of all things. It is only by a change of the physical nature that it can be done, by a descent of the highest light into this lowest part of Nature. It is here that the struggle is going on. The internal being of most of the sadhaks here, however imperfect still, is still different from that of the ordinary man, but the external still clings to its old ways, manners, habits. Many do not seem even to have awakened to the necessity of a change. It is when this is realised and done, that the Yoga will produce its full results in the Ashram itself, and not before.
30-4-1934
About Savitri | B1C3-11 Towards Unity with God (pp.31-33)