From the physical point of view, living beings are of six types -Pṛthvīkāyika, apkāyika, tejaskāyika, vāyukāyika, vanaspatikāyika and trasakāyika. On the basis of the availability of faculty of knowledge, they are of five types - one sensed beings, two sensed beings, three sensed beings, four sensed beings and five sensed beings.
The knowledge which is taking place through senses and mind has its relation with the body structure. Depending on the capacity of sensory and mental knowledge, the body structure of living beings is determined. On the contrary, based on the body structure, cognitive activity occurs. In Sthānāṅga sūtra there is a discussion regarding the body structure with sensory and mental knowledge. In the notes of Sthānāṅga given by Acharya Mahapragya the same fact is illustrated as per the table below -[1][*]
The five sensed living beings are of both types -samjñi (possessed with mind) and asamjñī (devoid of mind). The development of mind can take place only after obtaining the five senses. The structure of the body also has relation with mind.
One sensed beings have only one sense i.e. the sense of touch. These living beings are sthāvara beings. Two sensed living beings have the sense of touch and taste. Similarly, three sensed beings have the sense of touch, taste and smell. Four sensed beings have four senses viz. touch, taste, smell and vision. Likewise, the five sensed beings have the five senses i.e. touch, taste, smell, vision and audibility. The sequence of obtaining the senses is fixed. It means that one sensed beings would acquire the knowledge through the sense of touch alone. Two sensed beings etc. also have their determined hierarchy. The sense of taste can be obtained only after the attainment of touch and the sense of smell will be obtained after obtaining the sense of taste only.
The sequence of sense achievement is fixed. There is no possibility of any changes or reversal in it. Life in the trasa living beings is evident so there is no need of any postulations to affirm its validity. The division of trasa living beings has been extensively discussed in the āgamic
literature.