Thinking & Feeling

Thinking and Feeling are the Judging Processes, and when combined with an Attitude constitute the Judging Functions.
Those who prefer Thinking are called Thinkers.
Those who prefer Feeling are called Feelers.

Essence

Once again, before one can understand Thinking and Feeling, one must consider how the mind works.

As explained in the previous chapter on Sensation and Intuition, both sensations and pleasure & pain are qualia, raw experiences that cannot be divided. However, they differ in one important way: valence.
Sensory experience is neutral or inert. Pleasure & pain on the other hand are valent, that is, having to do with value and having motivating power.
Similarly, abstractions from sensory experience are also inert - for example E = mc^2 does not motivate anybody, though it may constrain their behaviour in some way. And abstractions from pleasure and pain - moral principles, rules, guidelines, and so on - they are valent just like pleasure pain.
[explanation of the evolutionary purpose of sensation vs. pleasure/pain]

Thinking involves a bias toward the neutral or inert. One who has an inclination toward Thinking prefers to divert their attention and mental energy toward matters that are not emotionally or morally charged, such as objects, mechanical or mechanic-like ideas and principles, and matters of truth and falsehood.
Feeling involves a bias toward the valent. One who has an inclination toward Feeling prefers to divert their attention and mental energy toward matters that are emotionally or morally charged, such as people, animals (things that 'have a soul' in a colloquial sense), beauty and ugliness, moral or aesthetic principles - above all, matters of goodness and badness.
It should be noted that both are trying to create value. Thinking merely seeks to do so in a more indirect manner - again, by keeping its attention on neutral matters, even if it is ultimately working toward some value-laden goal or in the spirit of some value. Valent things the only motivation we have, so no one does anything without desires, emotions, for value-purposes (as all purposes are derivative of value).

Vices and Virtues

The Vice of Thinking is its callousness. Due to Thinkers' lack of attention to the valent, they are liable to give too little concern to the wellbeing of others, and even themselves. They may also tend to be actively harsh, under the impression that anyone who objects is merely too sensitive to handle the truth (which is indeed often harsh...but not always, and not always necessarily). The truths they state may also often be unhelpful or even counterproductive, disconnected as they are from the value such truths may have.
The Virtue of Thinking is its detachment. It is not always right to be emotionally invested in what we're doing, especially when it comes to matters of truth and falsehood, which often do not appeal to our sensibilities. If one cannot detach, one is forever bound by one's biases.

The Vice of Feeling is its inability to detach. Things that seem nice are taken to be nice, and things that seem mean are taken to be mean. Thus, Feelers are prone to reject uncomfortable truths or falsehoods simply because they are uncomfortable, because to say them appears to them to imply immoral action, or because it simply requires them to take an attitude they consider callous or mean. This leads to enabling behaviour, for example not disciplining one's children.
The Virtue of Feeling its close connection with value. After all, all that we do is to live in accordance with what is good/against what is bad, or to create some good effect/prevent a bad one. Feelers are always in touch with this, unlike Thinkers who may forget it. Due to this, they also have an appreciation for not just morality, but aesthetics as well; it is Feelers who make the world beautiful.

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