Someone, a very insightful peripatetic spiritual seeker with direct experience of altered states of consciousness and a keen understanding of the human mind, was in my opinion the source of the oral traditions that lie behind the earliest layer of Buddhist literature in the Pali canon (such as the Aṭṭhakavagga, or
“Book of Eights”).
I'm comfortable calling this original person, whomsoever he was in purely historic terms, the Buddha (the "awakened one").
It's quite possible that the legend of Gautama, the sage of the Sakyas, was conceived to codify different strands of ascetic thought and practice - but I personally believe that the early teachings have certain continuities and peculiarities which make it more likely that they came from a unique historical personality, an intellectual systematiser and genius. At base, the Dharma (the Noble Eightfold Path, Four Noble Truths etc.) forms a coherent system of thought and practice.
It looks to me much more akin to a theory, in religious terms something like say Einstein's general relativity is in physics (once you strip the supernatural elements away, there's a kind of model left with some testable predictions made about suffering and its root cause that I think individuals can 'test' for themselves).
With that being said, unlike in the case of Jesus and Muhammad (whom serious scholars universally concur existed and about which certain facts are generally indisputable i.e. that Jesus was a Galilean Jew executed for sedition by crucifixion under the Roman procurator Pontius Pilate or that a man named 'the Muhammad' played a key role in uniting disparate Arab tribes and initiated the Arab conquests of the seventh century) there is
no proof at all, if by that one means contemporary or near-contemporary evidence, for Siddhartha Gautama's existence. As the scholar David Drewes noted in an article, ‘The Idea of the Historical Buddha’:
the Buddha… more than two centuries of scholarship have failed to establish anything about him...If we wish to present early Buddhism in a manner that accords with the standards of scientific, empirical inquiry, it is necessary to acknowledge that the Buddha belongs to [a] group [of mythological personages such as Agamemnon or King Arthur]’ (my italics)
In contrast, for Jesus (or indeed Muhammad (we have a few near-contemporary references to him by Syriac and Greek Christian authors and the Qur'an itself has surahs like 'Ar-Rum' that comment, reporter style, on events like as the Eastern Roman-Sassanian conflict) or John the Baptist but the below is for Jesus, in the interests of time):
- Stanton (2002, p. 145): Today nearly all historians, whether Christians or not, accept that Jesus existed and that the gospels contain plenty of valuable evidence which has to be weighed and assessed critically. There is general agreement that, with the possible exception of Paul, we know far more about Jesus of Nazareth than about any first or second century Jewish or pagan religious teacher.
- Burridge & Gould (2004, p. 34): "There's a lot of evidence for his existence."
- Ehrman (2011, p. 256-257): "He certainly existed, as virtually every competent scholar of antiquity, Christian or non-Christian, agrees, based on certain and clear evidence."
- Ehrman (2012, pp. 4–5): "Serious historians of the early Christian movement—all of them—have spent many years preparing to be experts in their field. Just to read the ancient sources requires expertise in a range of ancient languages: Greek, Hebrew, Latin, and often Aramaic, Syriac, and Coptic, not to mention the modern languages of scholarship (for example, German and French). And that is just for starters. Expertise requires years of patiently examining ancient texts and a thorough grounding in the history and culture of Greek and Roman antiquity, the religions of the ancient Mediterranean world, both pagan and Jewish, knowledge of the history of the Christian church and the development of its social life and theology, and, well, lots of other things. It is striking that virtually everyone who has spent all the years needed to attain these qualifications is convinced that Jesus of Nazareth was a real historical figure."
By the standards of scientific, empirical inquiry, Jesus and Muhammad existed according to the consensus of scholars and we know a few certain facts about their lives that lie beyond reasonable doubt. The Buddha does not pass that bar: there is no solid, factual, positivist, empirical evidence for the existence of the Buddha - but that doesn't mean he didn't exist. Absence of evidence is not necessarily evidence of absence and nor does the lack of a footprint in the historical record negate the immense value of the actual suttas attributed to the Buddha.