Self-knowledge is one of the hardest things to acquire, and it is precisely those who think they know themselves best who are most likely to be deceived: they think too much about themselves. You should get out of the habit of fixing your attention on yourself and constantly using the word “I” — “I think, I believe, I consider this right”.
Above all you must get rid of the notion that your opinion is worth more than that of other people. Suppose, for instance, that someone is very clever. If he displays his cleverness in the company of people who are not so clever, his behaviour will be very ill-timed; he will be doing it only to please his own egoism. He ought to adapt his response to the needs and capacities of others.
Source: Rudolf Steiner – GA 95 – At the Gates of Spiritual Science: Lecture XIV: Rosicrucian Training – The Interior of the Earth – Earthquakes and Volcanoes – Stuttgart, 4th September 1906