Quassim Cassam in The Philosophers' Magazine:
As you sit down to dinner at your favourite restaurant the waiter comes over and asks you what you would like to drink. You don’t find this a difficult question. A gin and tonic is (say) what you want and you know it’s what you want. So you place your order. Your irritating companion asks you how you know want a gin and tonic. A very strange question, no doubt, and probably a conversation stopper. Anxious to drink your gin and tonic you say you just know, and that is all there is to it.
Although your impatience with your companion’s question is perfectly understandable it raises an interesting philosophical question. On the face of it, any assertion is open to the challenge “How do you know?” For example, if you assert that it is raining in Mombasa then you can be asked how you know. So if “I want a gin and tonic” is a genuine assertion then it, too, is exposed to the question “How do you know?” You might not know the answer but there must be an answer. Equally, if you know you believe it’s raining in Mombasa there must be an answer to the question how you know that that is what you believe. Being asked for the answer over a gin and tonic might be a bit much, but if there is such a thing as knowledge of one’s own desires, beliefs, hopes, fears, and intentions it should be possible in principle to explain how such self-knowledge is possible.
In philosophy, rationalism is much impressed by the role of reasons in our mental lives and its account of self-knowledge is constructed on this basis. So if you are a rationalist you might be tempted to suggest that our beliefs and desires are normally determined by our reasons and so are knowable by reflecting on our reasons. For example, if your belief that it is raining in Mombasa is formed in response to the reasons in favour of believing this then you can know that you believe that it is raining in Mombasa by consideration of these reasons. By the same token, you can know that you want a gin and tonic by consideration of the reasons in favour of wanting one, as long as your desires are determined by your reasons. To put the point more simply, you can answer the question whether you actually want a gin and tonic by answering the question whether you ought rationally to want one.
Unfortunately, many of our desires are not determined by our reasons. If your doctor has told you to cut down on your drinking then you have a good reason not to want a gin and tonic but that doesn’t alter the fact that you want one. So consideration of what you ought rationally to want won’t be a good guide to what you actually want unless you are the kind of being whose desires are rationally determined.
More here.