math-modetext-mode
A common beginner's mistake is to forget \(...\)/$...$ around symbols that require math mode.
Why doesn't (La)TeX let people use isolated symbols in text mode by default?
I am sure there are at least historical reasons, but a learner with an eye for parsing matters will wonder about an explanation (not found in introductory literature).
Try:
\documentclass{article}
\usepackage{amsmath,amssymb}
\begin{document}
$ \lnot \square \mbox{Alarm} = \mbox{On} \wedge \mbox{DoorControls} = \mbox{Enabled}$
\end{document}
It is better to use siunitx for units.
\documentclass{article}
\usepackage{libertine}
\usepackage{siunitx}
\begin{document}
\begin{equation}
\SI{100}{\micro\meter}
\end{equation}
\end{document}
Best Answer
The real reason for not allowing
\thetain normal text has to do with how TeX deals with fonts.A TeX font has only 256 slots, while the mathematical symbols are many more. The command
\thetais, essentially, a four digit hexadecimal number:which carries a good deal of information: the leftmost digit is 0 and tells TeX that it's an ordinary symbol; the next digit tells TeX what font it should draw the character from and the final two tell where in the font the symbol lives (place
0x12=18). Up to sixteen fonts can be used for math symbols and they are not necessarily tied to the text fonts (actually font number 0 should be a text font, but it's an exception).TeX might have been designed so that a single command denoting a math symbol did the choice automatically, but the effects would be adverse in most cases.
One should also remember that math has its own spacing rules that TeX applies automatically. Letting people write math symbols in normal text would soon lead to poor typesetting.