Existentialism is a branch of philosophy that centers on the experience of thinking, feeling, and acting.

存在主义是哲学的一个分支，专注于思考、感觉和行动的体验。

I took a test in Existentialism.

我参加了一次存在主义的考试。

Existentialism is a philosophy of choice.

存在主义是一门关于选择的哲学。

I remember this paper I wrote on existentialism.

我记得我给存在主义写的这篇论文。

In this sense existentialism is optimistic.

在这个意义上说，存在主义是乐观的。

That is the first principle of existentialism.

这是存在主义的第一个原则。