The beauty of meditation can only be understood by meditation. Without the experience itself, it is all empty talk. Meditation, after an extended period of practice, gives rise to meditative concentration. You will experience this not only during meditation, but also during daily life. You see things very clearly, as though every single moment does not escape your knowing. Because of our practice, we understand the reality of being, and will not be turn by states. We stay calm and grounded in every situation. However, some people, at this point, may think or feel as if everything is the same. They may talk about emptiness and say that emotions, thinking etc. are not real. They may also speak in verses, or say some strange poems or sentences all the time, making people confused. What these people have is called meditation or Chan sickness.
What is this meditation sickness? Meditation is being stuck on emptiness, thinking that it is the result or the end of training. How do you spot someone who has this sickness? He might tell you that "Oh everything in the world is empty! The chairs and tables are empty. The clothes are empty!". They have caught the "emptiness" sickness, in which everything they see, they ascribe emptiness to it. For example, when someone close to this person has passed away, this person will deny every emotion that he or she may have, and say that "I am in the state of equanimity. I will stay calm and grounded in this". There is a difference between indifference and equanimity. Indifference is the result of meditation sickness. Indifference occurs due to ignorance.
This meditation sickness is not good. Why is this sickness instead of progress? Without a good teacher, one will be unable to notice that he has meditation sickness. Just like you need a doctor who knows about illnesses to diagnose you, and prescribe you with medicine. Meditation sickness hinders your practice. At the very least, you may be stuck at this stage for a very long time. Yet worse, you may gone off the Middle Way after having cultivated for a very long time, and fall off track. This will be very hard for the cultivator to go back on the right path. Hence one has to take note of meditation sickness.
Staying in this form of emptiness is not emptiness. It is a constructed form of emptiness, or false emptiness. Why is this so? Emptiness is originally empty itself, and does not need to be explained. Further explanation or description will ultimately fall into discrimination. It is like asking a sleeping person if he is sleeping. If he is sleeping how is he able to reply you that he is sleeping? If he replies you that "Yes I am sleeping", then in the first place how is he able to hear and talk to you when he is asleep? Emptiness is like that. Thinking that emptiness is something, one has fallen into duality, and that is meditation sickness. It is mistaking some temporary experience as real experience.
Back to the example of emotions. We are human beings. We have emotions. Indifference is denying the existence of the emotion, and therefore it is falling into false emptiness. However, equanimity is in fact an unwavering quality that will not be shaken by states. There is no trying with equanimity. If one tries to force equanimity, one falls into indifference. It is like breathing. If you try to breathe, you will not breathe naturally. Hence the development of equanimity comes with practice of meditation, with clarity of the mind. It is not with cognitive action whereby one tries to make it happen. To cure oneself from meditation sickness, one has to then be careful of one's own thoughts and behavior. Do not force anything out of your behavior or mind.