 Thomas was called by the aging Father Ferdinand Verbiest, who had just found himself appointed 'Vice-President of the Tribunal of Mathematics', (a very important and influential post in the Qing Empire of China), to join him in Beijing.After the death of Verbiest in 1688, Thomas took his place as the main mathematician and astronomical expert of China.For twenty years he was then a close adviser to the Kangxi Emperor who, beyond scientific questions, consulted him also on moral and religious matters.