He had been an eyewitness of and an actor in the events which he describes, but his testimony must be accepted with caution.
Of Greece proper he saw but little; it is by no means certain that he even visited Athens, and though he describes Corinth as an eyewitness, it is clear that he was never at Delphi, and was not aware that the ruins of Mycenae still existed.
After travelling from Lhasa to Peking with a lama mission he returned, again by Lhasa, to India, and was an eyewitness of the sack of Delhi by Nadir Shah in 1737.
He saw all the mechanical difficulties that had to be overcome in mining; he learned the nature and succession of rocks, the physical properties of minerals, ores and metals; he got a notion of mineral waters; he was an eyewitness of the accidents which befel the miners, and studied the diseases which attacked them; he had proof that positive knowledge of nature was not to be got in schools and universities, but only by going to nature herself, and to those who were constantly engaged with her.
Dr James Drummond's Inquiry into the Character and Authorship of the Fourth Gospel (1903) does not, by its valuable survey of the external evidence, succeed in giving credibility to the eyewitness origin of such a book as this is admitted to be.