In essence, his argument was that we need to simplify the process.

in 是介词，表示"在...之中"或"在...方面"。 essence 来自拉丁语 essentia，意思是"本质、实质、精髓"，指事物最基本的特征或性质。 in essence 是一个固定短语，意思是"本质上、实质上"，用来强调某事物的核心特征或根本性质。