Why does \nexists give an undefined control sequence error in TeXShop?

[MorningGlory]

Hello, everyone! First time poster. Quick question: I am working in TeXShop on a Macbook. When I type $\nexists$, I get an error message saying "Undefined control sequence" even though it is in the list of short course symbol tables available under the Help menu in TeXShop. $\not \exists$ works fine, but I would prefer the cleaner look that I should get with the $\nexists$ symbol. How can I fix this?

 

[jhae2.718]

Are you using the ams packages? I believe $\nexists$ is defined in amssymb. If not, try:

\newcommand{\nexists}{\not\exists}