this is an easy question, but I couldn't find the answer just from other similar questions.

A lot of people often get an error in the form:

```
LaTeX Error: Command \XYZ already defined.
Or name \end... illegal, see p.192 of the manual.
```

usually, the problem is that they are using something that is defined in 2 different packages, so LaTeX is confused about what to do with the command.

My symptoms are similar:

after lines

```
88 \begin{definition}
89 The distribution of a random variable $X$ is usually described by giving its {\bf distribution function}, $f(x) = P(X \leq x)$.
90 \end{definition}
```

my TeXShop is saying:

```
89:LaTeX Error: Command \c@T already defined.
Or name \end... illegal, see p.192 of the manual.
```

So, I tried to find where could command \c@T be defined. I'm only using

```
\usepackage{amssymb,amsmath}
\usepackage{changepage}
```

and I don't really see how they could both define command c@T.

I looked at the list of all commands in LaTeX, at http://www-sop.inria.fr/marelle/tralics/doc-c.html#cmd-citetype, and I haven't even found such command, but it looked like some kind of counting is the problem. So, I admit that I also have

```
\begin{document}
\newenvironment{definition}[1][Definition]
\newcounter{definition}
```

which might be the place where I defined something in the wrong way, but I don't see why wouldn't this problem come up in the previous definitions.

If anyone had patience to solve this old problem again and explain it to me, I would really appreciate it.

## Best Answer

What's apparently happening is the following: as @Ulrike remarks, your

`\newenvironment`

lacks its two mandatory arguments. It will instead pick up`\newcounter`

and`definition`

, since that is what follows.What this does is that when you call

`\begin{definition}`

, you inadvertently call`\newcounter`

, soturns into, with some technicalities omitted,

i.e.

and that's where your counters come from and that's why you get an error: you probably have to definitions that start with "T"!

(To backtrack and solve your problem: I think you really mean to look at the amsthm package and its

`\newtheorem`

command?)