Sentience is the capacity for basic consciousness — the ability to feel or perceive, to have sensations such as pain or nausea, or experience emotions such as fear and anxiety. A sentient animal is one that has the capacity to have (some or all) such feelings and conscious awareness.

The French philosopher, Rene Descartes, writing in the first half of the seventeenth century, held that animals act in a machine-like fashion without underlying consciousness or sentience. The more we find out about animals, the more this idea is disappearing. The evidence that higher animals (including mammals) have rich sensory, emotional and indeed cognitive capacities is now powerful. Such evidence makes it hard to deny that higher animals are sentient.
Rhesus macaques often show fear and defence responses – reflected in significant changes in 'normal' physiology – in the presence of laboratory personnel. Photo: Viktor Reinhart