Let’s start with Carl Jung, grandaddy of Swiss psychiatrist and psychoanalyst who founded analytical psychology. He’s responsible for much of this series, because of his theories on psychological types.
He introduced three areas or preferences, each of which are dichotomies: as in there are two ends of a pole, and your preference lies somewhere along that pole.
Introversion or extraversion are one area, sensing or intuition is another, and thinking or feeling is the third – and the one I’m going to dive into today.

Links mentioned:

Great ‘splainer vid on Carl Jung
Thinking/Feeling Quiz