mind
The German word 'Geist' translates to 'mind' in English. It is used to refer to the human consciousness, the intellect, and the capacity for rational thinking. It can also refer to a person's spirit or soul. In German, 'Geist' is often used in philosophical or psychological contexts to discuss the nature of human thought and consciousness.
The spirit is willing, but the flesh is weak.
This is a proverb, originally a quotation from the Bible (Matthew 26:41), used to mean that somebody's determination is stronger than their physical state.
He could not understand the spirit of the times.
This sentence is expressing someone's inability to comprehend the current cultural or societal mood or attitude. The spirit of the times (der Geist der Zeit in German) is a phrase also known as 'Zeitgeist'.
Spirit without matter is as unimaginable as matter without spirit.
In this sentence, the speaker is making a philosophical statement about the interconnectedness and mutual dependence of physical/material and spiritual aspects of existence.