Question on a sentence:

[Bob20020]

I found this

Why is "ja selber" needed there?

Das glaubst du ja selber nicht.

It seems to mean "even" but why is it not "sogar" or one of the other many words that mean "even"?

Viel'n dank

[Stusstrupp]

When "selbst" precedes the noun to which it relates, it is being used as an adverb, meaning "even":

Das glaubst selbst du nicht / even you don't believe that.

Selbst das glaubst du nicht / Even that you don't believe.

When "selbst" or "selber" comes after the noun, it is being used as a demonstrative pronoun like my-, your-, him/her/it- etc. "-self":

Das glaubst du selbst nicht = Das glaubst du selber nicht / You don't believe that yourself.

[Bob20020]

Why is "ja" there?

[Stusstrupp]

Here, "ja" is a modal particle used to emphasize the verb, like the adverb "surely" does in English:

Das glaubst du ja selber nicht / Surely, even you don't believe that.