Nietzsche and Camus come to mind. They don't exactly address emotional apathy as a philosophical problem. But they both develop philosophical positions that, in quite different ways, combine intellectual argument with emotional engagement. You might have a look at Nietzsche's Thus Spoke Zarathustra and Camus' The Rebel.

One book you might be interested in is Descartes' Error: Emotion, Reason, and the Human Brain by Antonio Damasio (although Damasio is a neurologist, not a philosopher).

Only if you define "rationality" in a way that makes it opposed to emotion. But for a lot of reasons, that would be a dubious definition.

For one thing, we have reason to believe that intelligent decision-making isn't disconnected from emotions. There's been a good deal of work on this topic by philosophers and scientists, but one well-know place to start is with Antonio Damasio's book Descartes' Error. It turns out that the emotional centers in the brain have an important role to play in helping keep us on the rails.

We can add: other things being equal, it doesn't sound rational to choose a life that makes it less likely that we'll be happy and fulfilled. But for most of us, a good deal of what makes life meaningful is bound up with our emotions. In a perfectly obvious sense of "rational," it's rational to seek love, let ourselves cry in the face of tragedy and open ourselves to joy. A concept of "rationality" that ruled all this out would be poor and perverse.

There is a lot of really interesting philosophical work currently being done in the area of forgiveness (in fact, the 2008 Annual Meeting of the American Catholic Philosophical Association next November is planned around the theme of forgiveness; and its consideration won't be limited to religious teachings). So what I say here should not be taken as the last word, or even anywhere close to it.

The wronged party is indeed owed something ("has an entitlement from") the wrong-doer; voluntarily foregoing this entitlement is precisely the essence of forgiveness. If this is so, it is simply inaccurate to tell someone, "You must forgive." Forgiveness is not an obligation, or else it wouldn't really be forgiveness. It must be a freely chosen act.

I would say that forgiveness is NOT an emotion, but rather a deliberate movement of the will -- a free choice to waive the entitlement owed by the wrong-doer. Sometimes that entitlement will consist in compensation, material or non-material, sometimes in a loss of trust, a sense of offense, or a grudge.

The accounts of the value of forgiveness with which I'm most familiar are in the vein of virtue ethics, i.e., what we ought to do is to develop in ourselves a certain kind of character, or habitual way of acting. A forgiving nature (assuming it is cultivated by autonomous choice and not by coercion or shame) is virtuous, while hard-heartedness makes us something less than we ought to be.

Feeling pain is no more authentic than thinking that you're feeling pain. It is just that the two ways of accessing the experience of pain are different. When we feel something - pain, joy - we may be not aware that we are feeling it, whereas thinking that you're feeling pain or joy is a conscious experience in which our conceptual apparatus is mobilized. But this doesn't mean that feeling is a more authentic experience than thinking you're feeling. There are experiments that show that injured people feel pain in their amputated limbs even if they know that they cannot feel it anymore. That is to say that you may be deceived by your feelings as well as by your thoughts about your feelings.

As for discerning how we feel as opposed to how we think we feel, I would say that a sensorial experience is always underdetermined. We feel the gap between the raw experience and its conceptualisation according to our previous experiences, our cultural background and what we know about the world and ourselves. Feeling love may be described by a Shakespeare's sonnet or by a simple "Wow!". It is very difficult to disentangle what belongs to the "authentic" experience and what belongs to our way of making sense of it.

The anglo-american philosophical tradition has not been very kind to the emotions until relatively recently, when there has been an upsurge of support for the idea (latent, however, in Aristotle) that emotions can have cognitive content - they can tell you stuff about how the world is. The emphasis has rather been on the opposing dynamic of emotion - their ability to disrupt rational processes and so constitute an obstable to knowledge. Certainly, emotions can be an obstacle to knowledge; but it is important not to underestimate their positive cognitive power too. In the early eighties, feminist philosophers started writing about the role of emotions in telling you important things about your social experience: your anger that you are treated a certain way might be telling you something, namely, it's unjust to be treated this way. If you are living in a social-conceptual environment that offers you no tools to making sense of your experience as one of mistreatment, your anger is vulnerable to seeming misplaced, hysterical, when in fact the contrary is the case - your anger is a rational response to the treatment you are receiving, but collective forms of interpretation have not caught up yet. In cases like this, emotions can be a crucial cognitive resource for social and ethical change. And more recently there has been a lot more work vindicating the cognitive contribution of emotion.

Cases like the anger example above support the idea that emotion can have not only intentional content (it is directed to the world, it is about the world) but also cognitive content (it represents the world as being a certain way, and thus permits of truth or falsity). The anger example is primarily a case of ethical/political knowledge, but we can easily imagine more plainly empirical versions where what our emotional responses are telling us concerns, for instance, someone else's psychological states. For example, an emotional response of distrust or suspicion can tell us (defeasibly, of course - like any evidence) that someone has malevolent intentions towards us. These sorts of human response are crucial indicators for us in our social dealings with other people. At the minumum, they provide us with evidence, and if suitably reliable, can provide for empirical knowledge of others' psychology (intentions, attitudes). But more than this, rather than the emotions constituting evidence on which knowledge might be based, perhaps the emotions can themselves constitute the knowledge. Perhaps your anger, your suspicion, your trust, your sympathy can be the form of your cognitive grasp of the relevant facts.

"Knowledge" might be divided into four types: (1) theoretical knowledge (knowing that X); (2) practical knowledge (know how); (3) familiarity (knowing someone); and (4) moral knowledge (knowing what's right).

1. Emotion alone doesn't seem able to produce theoretical knowledge. In fact, emotion often obstructs it.

2. Emotion alone can't make it possible for us to know how to do something--e.g. drive a car or play the violin. But it can be a necessary condition for us knowing how, for example, to play music well or for knowing how to manage people psychologically (as an effective manager, parent, or politician might know how to do).

3. Emotion might be the result of familiarity, but knowing someone isn't made possible by emotion alone.

4. The very idea of moral "knowledge" is a strange one, but one might say that knowing what the right thing to do in a given situation might be said to be determined through feeling. But I doubt it would make sense to say that emotion alone yields moral determinations.

There is another way emotion might be said to function relevant to your question. Philosophers like Martin Heidegger, Jean-Paul Sartre, and Soren Kierkegaard have held that certain specific emotions--like anxiety and a specific form of nausea--are part of the disclosure of certain very general features of human existence, in particular its contingency, its finitude, and our freedom in the face of it. Calling this knowledge might be a bit of a stretch, but it wouldn't be entirely misplaced.

It really depends upon what it is that one is supposed to come to know this way--and it will also depend upon just what one takes the requirements of knowledge to consist in.

Some epistemologists have argued that we have a kind of privileged access to knowledge of our own mental (including emotional states) themselves. These philosophers would think that at least one sort of knowledge we could attain through emotions was knowledge of those emotions themselves--knowledge that we were in such-and-such a state at a given time (for example, knowledge that I am angry right now, or sad). But others do not think that we necessarily know our own states in any privileged way--we might really be angry, but not know that we are, or we might think we are angry, but actually not really be.

As for other sorts of knowledge, such as knowledge of the world outside of our own consciousness, I am inclined to think that we cannot "acquire knowledge through emotions only." As important as the emotions are in our lives, I do not think they are very reliable or clearly informative as sources of cognition (such as knowledge).

Emotion-less or emotion-free creatures/beings have been explored in science fiction, including Stanley Kubrick's "2001" and "Star Trek." See what Wikipedia says about the 1956 B&W movie, "Invasion of the Body Snatchers," at http://en.wikipedia.org/wiki/Invasion_of_the_Body_Snatchers.

And it is not only non-cognitivists who would believe that it would make a moral difference if humans did not experience certain emotions.

For example, suppose that I were a cognitivist hedonist, act Utilitarian. I believe that one ought always to perform the act that produces the greatest balance of pleasure over pain, and I believe that that moral principle is true independently of my or anyone else’s commitment to it. For me, it would make a great deal of difference what sorts of emotions human beings experienced in different circumstances, since the amount of pain or pleasure that anyone feels as a result of my actions will depend, at least in part, on their emotional attachments.

Or suppose that I am a cognitivist virtue theorist of the following variety: I believe that I ought to live my life in such a way that I live a good and meaningful life, and I believe that a corresponding normative principle applies to everyone else. They too ought to live their lives in such a way that they live good and meaningful lives. Let’s suppose also that I believe that, to achieve this end, I need to cultivate certain sturdy character traits. I call such traits "virtues". Given my current emotional dispositions and the emotional dispositions of my fellow human beings, some of these character traits are ones that we would recognize as moral: justice, beneficence, friendliness, loyalty. However, if human beings changed– if they no longer were able to form any emotional attachments to other people, but were all content to live in isolation, indeed resented any intrusion into their individual lives, then again, what character traits I should cultivate in myself to live a good and meaningful life would change. On this view, then, depending on human nature, including our emotional nature, different traits would count as virtues.