this book , the myth of the historical buddha , was more an invective against theravada buddhism than an exposition of mahayana , holding firmly to the line that there was indeed no single source for buddhism , that there had never been a person called gautama siddhartha , that buddhism was a natural development from the veda and upanishads , and that its source was a string of excellent brahman meditators , developing the four noble truths and the eightfold path somewhere around 500 bce .