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.
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.
Contemporary observers agree that the disease was introduced from the East; and one eyewitness, Gabriel de Mussis, an Italian lawyer, traced, or indeed accompanied, the march of the plague from the Crimea (whither it was said to have been introduced from Tartary) to Genoa, where with a handful of survivors of a Genoese expedition he landed probably at the end of the year 1347.
He had been an eyewitness of and an actor in the events which he describes, but his testimony must be accepted with caution.