for knowledge of the external world, whether of things or of people, we continually come back to our five senses in the end. ( ⓐ ), if a great mass of data relevant to social science is sensory, we have also to admit an important collection that is not ― namely the whole body of primary mental or psychological experience.

다음 중 ( ⓐ )에 들어갈 가장 적합한 표현은?
① for example
② in other words
③ furthermore
④ nevertheless