Objects and Doors - Six classes of objects

The Buddha taught about objects, experienced by cittas through different doors, in order to cure people of their blindness. When we study the teachings we learn that there are six classes of objects (in Pāli : ārammana), which can be known by citta.
The first class is : visible object or rūpārammana. The object which is experienced through the eye-door can only be the kind of rūpa which is visible object. We can call it visible object or colour, it does not matter how we name it, but we should know that it is just that which is visible, which appears through the eyes. Visible object is not a thing or a person we may think of. When we think that we see a tree, animal or man, we think of concepts and there is not the knowing of visible object.
The second class of ārammana is sound, or saddārammana.
The third class is smell, or gandhārammana.
The fourth class is taste, or rasārammana.
The fifth class is the object which is experienced through the bodysense, photthabbārammana. This object comprises the following rupas: Solidity or the 'Element of Earth' (in Pāli : pathavī-dhātu), which can be experienced as hardness or softness. Temperature or the 'Element of Fire' (in Pāli : tejo-dhātu), which can be experienced as heat or cold. Motion or the 'Element of wind' (in Pāli: vayo-dhātu), which can be experienced as motion or pressure. Solidity (earth), cohesion (water), temperature (fire) and motion (wind or air) are the 'four principal rūpas' (mahā-bhūta-rūpas). Cohesion (apo-dhātu) can' t be experienced through the body-sense. When we touch water the characteristics of hardness or softness, heat or cold, motion or pressure can be directly experienced through the body-sense. The characteristic of cohesion can be experienced only through the mind-door; it is included in the sixth class of ārammana, the dhammārammana.
Dhammārammana comprises all objects which are not included in the first five classes. These can be experienced only through the mind-door.