Short answers, pulled from the story.
"Historian" became a professional occupation in the late nineteenth century, emerging out of research universities in Germany. The professionalization process then spread to Japan, the Netherlands, Italy, Spain, Russia, Scandinavia, Greece, Romania, and Latin America, with each country adapting the German model with local modifications.
Herodotus of Halicarnassus (484 - c. 425 BCE) was called the "father of history" by Cicero. Herodotus wrote The Histories, the earliest known piece of critical historical writing, and distinguished himself by attempting to evaluate source reliability and by traveling extensively to gather firsthand accounts.
The trial established the first legal benchmark for what constitutes an objective historian. Justice Gray relied on expert witness Richard Evans's analysis, and Wendie E. Schneider later distilled seven criteria in the Yale Law Journal, including treating sources with appropriate reservations, not cherry-picking evidence, clearly indicating speculation, and taking the motives of historical actors into consideration.
The Whig interpretation of history presents the past as an inevitable progression toward greater liberty and enlightenment, culminating in modern liberal democracy and constitutional monarchy. Herbert Butterfield coined the term in his 1931 book The Whig Interpretation of History and argued against it, calling instead for a historical sensibility that searches for the unlikenesses between past and present.
Leopold von Ranke was a nineteenth-century German historian considered the founder of modern source-based history. He implemented the seminar teaching method, insisted on archival research and primary sources, and published his first book, the History of the Latin and Teutonic Peoples from 1494 to 1514, in 1824, drawing on memoirs, diaries, government documents, diplomatic dispatches, and eyewitness accounts.
Three reasons are widely accepted in the field: a historian's interests inevitably shape which information they use or omit, all sources carry their own biases, and historians are products of their culture, concepts, and beliefs. It is broadly acknowledged that "strict objectivity is epistemologically unattainable for historians" and that specific canons of historical proof are neither widely observed nor generally agreed upon.