E refers to extroversion and extroversion.

This statement comes from Jung's MBTI Sixteen Personality, in which E and I are the first dimension. I think "the source of strength" may not be the distinguishing standard between E and I. E can also come from the inside, not always active, and I can also come from the outside.

The difference is that E is good at externalization and I is good at internalization. E can externalize the inner things, such as turning an idea into a visible thing or expanding it. I can internalize external things, such as turning external people and things into my own words, or I am good at finding several or one intimate object in the crowd.