From a letter to Winnie Mandela in Kroonstad Prison, dated 1 February 1975, see pages 211-212 of Conversations with Myself by Nelson Mandela.
…the cell is an ideal place to learn to know yourself, to search realistically and regularly the process of your own mind and feelings. In judging our progress as individuals we tend to concentrate on external factors such as one’s social position, influence and popularity, wealth and standard of education. These are, of course, important in measuring one’s success in material matters and it is perfectly understandable if many people exert themselves mainly to achieve all these. But internal factors may be even more crucial in assessing one’s development as a human being. Honesty, sincerity, simplicity, humility, pure generosity, absence of vanity, readiness to serve others – qualities which are within easy reach of every soul – are the foundation of one’s spiritual life. Development in matters of this nature is inconceivable without serious introspection, without knowing yourself, your weakness and mistakes. At least, if for nothing else, the cell gives you the opportunity to look daily into your entire conduct, to overcome the bad and develop whatever is good in you. Regular meditation, say about 15 minutes a day before you turn in, can be very fruitful in this regard. You may find it difficult at first to pinpoint the negative features in your life, but the 10th attempt may yield rich rewards. Never forget that a saint is a sinner who keeps trying.