Originally, the words "dissertation" and "thesis" had separate meanings. When, at ancient universities, the professor had completed his lecture, there would be a disputation. During this, students could take up certain points and argue them. The position that one took during a disputation was the thesis, while the dissertation was the line of reasoning with which one supported it.
Doctoral ceremony at Leiden University (7 July 1721).
Cover page to Søren Kierkegaard's university thesis (1841).
The cover of the thesis presented by Claude Bernard to obtain his Doctorate of Medicine (1843).
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Thesis Facts for Kids. Kiddle Encyclopedia.