Augustus De Morgan is a 19th-century mathematician and logician most famous for the De Morgan laws, even though he did not formulate them. He did, however, contribute (along with George Boole, Charles Peirce, etc) to the work of making logic into a mathematical discipline.

Quotations

‘Imagine a person with a gift of ridicule. [He might say] First that a negative quantity has no logarithm; secondly that a negative quantity has no square root; thirdly that the first non-existent is to the second as the circumference of a circle is to the diameter.’ (That is, $\ln(-1)$ and $\sqrt{-1}$ are both imaginary, but their quotient is $\pi$.)

‘I end with a word on the new symbols which I have employed. Most writers on logic strongly object to all symbols. […] I should advise the reader not to make up his mind on this point until he has well weighed two facts which nobody disputes, both separately and in connexion. First, logic is the only science which has made no progress since the revival of letters; secondly, logic is the only science which has produced no growth of symbols.’