Sentient beings and Dukkha in Mahayana Buddhism

A sentient being possesses a mind, whereas an automaton does not. Any animal whose survival strategy and behavior appears to depend on the avoidance of suffering (rather than mere reflex actions) should be assumed to be sentient. Apart from being unethical, it is regarded as bad karma to deliberately inflict suffering on any sentient being (Buddhists believe that cruelty results in the resultant suffering being experienced several times over by the one who inflicted it ). It is particularly bad karma to enjoy inflicting suffering.

More specifically, in Buddhist philosophy a sentient being is one who is aware of dukkha, and is capable of experiencing the qualia of suffering and happiness. A sentient being experiences its inputs (perceptions) and outputs (actions), in contrast to an automaton where no subjective states occur, and all meanings have to be assigned to inputs and outputs from 'outside the system'.
There is a Western school of philosophy known as dualism which claims that only humans experience qualitative mental states, and animals are automata with no subjective experiences, so it doesn't matter what you do to them. This view originates from the old Christian dogma that humans are qualitatively different from all other animals (because only humans have immortal souls). Dualism is not only contrary to Buddhist teachings, but also to the modern scientific understanding of evolution.
The functions of sentience
In Buddhist philosophy the mind of a sentient being is not a product of biological processes, but something primordial which has existed since beginningless time and which will be drawn into another body once the present one has died. The mind is capable of exisiting independently of the body, but an unenlightened mind finds this situation (known as the Bardo) unstable and is drawn into (rather than seeks) another body. In biological terms the mind and body form a symbiotic association.

But all biological systems are subject to evolution, and any adaptation or feature must have some selective benefit for the organism that possesses it. So what does the biological partner (the body) gain from the symbiotic association with mind? The obvious explanation is that it will have an improved chance of surviving to propagate its genes over any mindless competitor which is not deterred by pain or motivated by pleasure.

And what does the mind gain? Usually little or nothing. When the life of the biological partner comes to an end, it has to endure suffering and then leave its home, being able to take nothing with it. It must then enter the nightmare state of the bardo and soon after find a new body. In Buddhist terminology these minds are wanderers or migrators in samsara (the realm of perpetual death and rebirth, suffering and craving).
Perhaps the relationship between mind and body is more one of parasitism than symbiosis. The biological body gets to propagate itself. But the mind has to endure dukkha - the ever-changing experiences of craving, suffering and attachment that the body imposes upon it in order to force it to do what is necessary for survival, competition and reproduction.
The only way that the mind can escape samsara - being endlessly captured and used by biological systems - is to escape from the recurrent process of death, attraction to a body, and rebirth. Training in the Buddha's Dharma is stated to be the path to individual liberation, which is why a mind born into a human body is regarded as extremely fortunate - since only humans can understand Dharma. Nevertheless the minds of all sentient beings are believed to possess Buddha seed and are capable of progression.
In some Buddhist traditions it is believed that it is possible to help animals by reciting mantras to them (typically by whispering them in their ears). This will create a karmic connection between the animal and the Buddha of the mantra, causing the creature to be reborn in a situation where it will meet with the Dharma. In addition, Mahayanists believe that there are Bodhisattvas who are working for the salvation of all sentient beings, and have vowed not to enter Nirvana (the bliss of liberation) until they have rescued everyone from suffering.
- Sean Robsville