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In mathematics, "therefore" is represented as a triangular set of three dots. This may have developed from alchemy: some alchemical formulas involved burning a substance down into a fine white powdery ash, which resembled salt. The "therefore" symbol may have stemmed from this process: something is burned, therefore we have a "salt" (as far as alchemy is concerned; not the NaCl kind). Further, the alchemical symbol for salt is in fact a set of three dots. This roughly corresponds to the symbol's significance in mathematical proofs. 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:
A handful of other logic symbols originated with this one: for example, the because symbol is an upside-down version of it, and the ratio and proportion symbols are also sets of dots (resembling a colon and double-colon, respectively).
Swap says re Therefore: It's a funny thing that in modern mathematics the symbol is practically no longer used, where by "modern" I mean "university". It belongs to a logical style of argument that dates back to Aristotle, and ever since the so-called crisis in foundations of mathematical logic, other symbols became more fashionable, such as the "implies" symbol, ⇒ | 
