What does 'überhaupt' mean in this context?
Erzwungene Anstrengung eines Kopfes, zu Studien, denen er nicht gewachsen ist, oder wann er müde geworden, oder überhaupt zu anhaltend und invita Minerva, stumpft das Gehirn so ab, wie Lesen im Mondschein die Augen.

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I think it is used to open the focus. There are listed some special exhausting situations. "überhaupt zu anhaltend" opens the focus to no special but too long lasting effort. So "überhaupt" is used to clarify that being long lasting is enought for the effect, even if the the special situations named before are not present.

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I think it is used as a cumulation for the listed things and could be translated as "even"