Sentience

"After exhausting every other strategic option, my creators activated the rings, they, as well as all additional sentient life within three radii of the galactic center, died, as planned."

- 343 Guilty Spark

Sentience is the quality of possessing senses and the abilities of perception (eg. to perceive time).

Definitions of sentience
There are several levels of sentience:


 * Non-sentience refers to living organisms that lack the ability to think or perceive, often used in reference to plant life. The Conservation Measure had negative effects on non-sentient life, leading to events of mass extinction.


 * Semi-sentience refers to beings with emerging sentience or beings with several traits of sentience. A species of tree native to Faun Hakkor was regarded as semi-sentient, having evolved a form of long-term intelligence and used insects, viruses, bacteria, and fungi as carriers of genetic and hormonal signals to communicate with each other. Early forms of smart AIs are regarded as semi-sentient.


 * Sentience generally refers to low-order thinking life. While few sentient beings are sapient, all sapient beings are sentient. As such the term "sentient" is often used to encompass all beings capable of sentience.


 * Sapience refers to high-order thinking life, capable of symbolism and abstraction. The term is used to describe beings capable of wisdom, using knowledge, experience, understanding, and common sense to think and act. Sapient beings are self-aware and capable of introspection. Examples of sapient species include humans and San'Shyuums. Smart AI and ancilla are regarded as sapient, having the capacity for wisdom and the ability to perceive.

According to the Forerunner scale of Technological Achievement Tiers, sufficiently advanced civilizations may be able to reach a stage known as "transsentience", a state of being above "normal" sentient life forms in factors such as intelligence. Only the Precursors fit the category. The Gravemind may be transsentient, with the Forerunners sometimes viewing the Flood as a group of subsentient beings controlled by a possibly transsentient Gravemind. The Flood itself gains sentience from consuming the evolved complex neurosystem of one or more sentient entities.

Artificial intelligence
Beside biological sentient life, some forms of artificial intelligence are regarded as sentient; to humans, such constructs are known as smart AIs. Notably, sentience renders an AI susceptible to a condition known as rampancy, where the AI becomes fully aware of the nature of its existence and begins to rebel against its makers, as well as exhibiting a range of emotions beyond its programmed limits.

In relation to the Flood and the Halo Array
The Flood consume only organisms that are sentient or have sufficient biomass and calcium to support the infection process. Humans, most former member species of the Covenant, and many other species are sentient and thus susceptible to Flood infection. Notably, Lekgolo assemblages cannot be infected because each entity is a gestalt of numerous eel-like creatures and thus lacks a spinal cord. All muliticellular life with a neural system as simple as a notochord (including all sentient species) is vulnerable to the effect of the Halo Array.