Usage of the symbol in a mathematical context dates back to at least 1659, from Teusche Algebra by Johann Rahn (source). How to insert the symbol: Unicode: 0x2234 UTF-8: 0xE2, 0x88, 0xB4 HTML entity: ∴ (∴) LaTeX: \therefore (after \usepackage{amssymb}) Using the Adobe...