The concept of naturalness in animal welfare - the extent to which animals can express natural behaviours and live in environments resembling their evolutionary context - has generated significant philosophical and scientific debate. Five Freedoms (1979) and later Five Domains frameworks include behavioural needs and positive experiences alongside freedom from suffering. Critics of industrial livestock housing argue that prevention of species-typical behaviour is intrinsically harmful regardless of clinical health outcomes. Defenders of indoor systems argue welfare is measurable through health and production metrics, not environmental aesthetics. Research using preference tests and cognitive bias paradigms demonstrates that animals are motivated to perform natural behaviours independently of clinical health benefits. Consumer attitudes increasingly reflect naturalistic welfare preferences, driving market differentiation toward pasture-based and enriched systems. Welfare philosophy has shifted from purely suffering-prevention to encompass positive welfare states and flourishing - a framework more demanding of natural behaviour provision than clinical health assessment alone.