Explain why lifelong synaesthesia is generally considered a normal variant of perceptual experience rather than a sign of illness.

Lifelong synaesthesia is a stable, consistent, automatic pattern present since childhood, reliably triggered by a genuine stimulus, and is not associated with distress, functional impairment, or any of the other features that would typically accompany a psychiatric or neurological disorder. Because it represents a consistent, predictable way that one particular person's sensory systems are wired together, rather than a departure from a previously normal baseline or evidence of an underlying disease process, it is understood as a normal variation in how perception can be organised, comparable to other stable individual differences in perceptual experience rather than a symptom requiring explanation by illness.