In the Nikayas we come across three closely related terms of citta, mano and viññāna, as a designation for the ‘mind’ in general, but in some other contexts also for its three distinct aspects. Among them citta is typically used in a singular form and in a sense of ‘thought’, but also consciousness in general. Although the Buddha hasn’t formally defined it, based on the usage of the term in various suttas, it turns out that citta‘s role is cognition of an object. Thus it has a central role in intellectual and moral behavior of an individual: “The world, bhikkhu, is led by the mind; it is dragged around by the mind; when the mind has arisen, [the world] goes under its control.” (“cittena kho, bhikkhu, loko nīyati, cittena parikassati, cittassa uppannassa vasaṃ gacchatī”ti) (AN 4:186, transl. B. Bodhi).
On the other hand, Abhidhamma approach was the one of much greater elaboration, taking us to the microscopic level regarding phenomena of mind and matter. And that particularly applies to the citta. As Ven. Nyanaponika describes it: “One of the Abhidhamma’s most important contributions to human thought, though still insufficiently known and utilized, is the analysis and classification of consciousness.”
First of all, citta is considered as one of the four ultimate realities (paramattha dhammā) and than it’s being dissected further on. Abhidhamma explains citta as a series of momentary acts of consciousness of various types, although their rapid succession creates an impression that we are dealing with a single act of consciousness. Abhidhammikas employed several criteria when classifying types of cittas. One is by plane (bhūmi) of consciousness, of which there are three mundane and the one supramundane: 1. sense-sphere consciousness, 2. fine-material-sphere consciousness, 3. immaterial-sphere consciousness and 4. supramundane consciousness. These planes are not identical with the planes of existence of the same name, although each type of consciousness is typical for the plane of consciousness of the same name.
Another, very important criteria of classification is by nature (jāti): 1. unwholesome, 2. wholesome, 3. resultant, and 4. functional. As every consciousness is accompanied by various mental factors, (cetasika), unwholesome consciousness (akusala citta) is the one associated with one of the three unwholesome roots: greed, hatred, and delusion. On the other hand, wholesome consciousness is characterized by the absence of these unhealthy concomitants and supported by the positive ones: generosity, loving-kindness, or wisdom. Both of these two types of cittas direct our volitional actions, which means they are producing kamma. This brings us to the third type of consciousness, which is resultant to the ripening of the kamma. The last type accompanies those activities which do not produce kamma, nor its results.
Relying on the material in the Nikāyas, Abhidhamma finally developed a very elaborate classification of cittas, which lists as much as 89 mundane and 8 (or 40) supramudane types of consciousness.