The Buddha, as a teacher, was adamant about the power of action, and focused special attention on what skillful actions could accomplish in bringing about a true and unchanging happiness. So it’s only natural to want to know what he himself did, and what he was able to accomplish through his actions. An account of his life allows people who are not yet convinced of his awakening to assess him as a teacher and the value of his teachings, to see if he practiced what he taught and if it was a good thing that he did so. It also allows those of us who are convinced of his awakening to learn by example what the Buddha regarded as skillful action and to gain inspiration from the way he lived his life.
This is why many biographies of the Buddha have been composed over the millennia. Ironically, the most authoritative source for learning about the Buddha’s accomplishments, the Pāli Canon, contains chronological accounts of only a few parts of his life: his birth, his quest for awakening, the first year or so immediately after his awakening, and then the last year of his life. There is a wealth of material on the intervening years, but by and large it’s fragmentary. There are a few long narrative accounts, but although they scrupulously tell where the events they narrate took place, they make little or no attempt to indicate when. Only rarely does a passage state that the Buddha was young or old at the time of the incidents it reports. The early parts of the Canon relate some of the important events in the very early and late periods of his life—such as his return to his family, the ordination of his step-brother, the death of his two chief disciples, and the death of one of his major supporters, King Bimbisāra —only in bits and pieces. They say nothing of his interactions with his wife, either before or after his awakening, and don’t even mention her name.
Most biographers have thus chosen to fill in the blanks. This is a tendency that began in the early centuries of the tradition, as the genres of literary biography and the epic developed in India, and Buddhists felt a need to provide their Teacher with an aesthetically and dramatically satisfying life in line with these genres. Some of these accounts found their way into later additions to the Canon, such as the Apadānas. Later still, the authors of the Pāli commentaries— systematic explanations of the Canon that postdate the Canon by many centuries—added even more dramatic incidents in their account of the Buddha’s life, along with a detailed account of where the Buddha spent his various Rains retreats every year from the time of his awakening to that of his passing away. There is, however, no clear evidence in the early parts of the Canon to substantiate these additions to the Buddha’s story.
But even though these parts of the Canon don’t provide enough evidence for a strictly chronological biography, they do contain ample material for a thematic one, i.e., an account that focuses on themes and activities that ran throughout the Buddha’s life.
This reflection by Ajaan Geoff is from the book, Noble Warrior, (pdf) pp.4-5.