Tamas (Sanskrit: तमस् tamas "darkness") is one of the three Gunas (tendencies, qualities, attributes), a philosophical and psychological concept developed by the Samkhya school of Hindu philosophy. The other two qualities are rajas (passion and activity) and sattva (purity, goodness). Tamas is the quality of inertia, inactivity, dullness, or lethargy.
In Samkhya philosophy, a guṇa is one of three "tendencies, qualities": sattva, rajas and tamas. This category of qualities have been widely adopted by various schools of Hinduism for categorizing behavior and natural phenomena. The three qualities are:
Action that is virtuous, thought through, free from attachment, and without craving for results is considered Sattvic; Action that is driven purely by craving for pleasure, selfishness and much effort is Rajasic; Action that is undertaken because of delusion, disregarding consequences, without considering loss or injury to others or self, is called Tamasic.
In Indian philosophy, these qualities are not considered as present in either-or fashion. Rather, everyone and everything has all three, only in different proportions and in different contexts. The living being or substance is viewed as the net result of the joint effect of these three qualities.
According to the Samkya school, no one and nothing is either purely Sattvic, Rajasic or Tamasic. One's nature and behavior is a complex interplay of all of these, with each guna in varying degrees. In some, the conduct is Rajasic with significant influence of Sattvic guna, in some it is Rajasic with significant influence of Tamasic guna, and so on.