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I don't happen to believe in an interventionist God, but I wouldn't criticise those who do, and to be honest, I do sometimes pray for external thing to happen, for example to alleviate someone's suffering. It can give the feeling that you have 'done something', when there is nothing else one can do.
What Rowan Williams reveals in his connection to Buddhist prayer is that it can be thought of as a focused expression of the sentiments of yearning, commitment and appreciation. It is, however, distinguished by the fact that Buddhism locates the divine within the life of the individual practitioner. The purpose of Buddhist prayer is to awaken our innate inner capacities of strength, courage and wisdom rather than to petition external forces.