Bernard Bolzano (1781-1848) presents Bolzano’s theorem: a continuous function which is negative at one point and positive at another point must be zero for at least one point in between.
Editors. "Bolzanos theorem". The Literary Encyclopedia. First published 01 May 2009
[http://www.litencyc.com/php/stopics.php?rec=true&UID=6245, accessed 19 June 2013.]