Opinion. Cats are farsighted and when they focus on one object all other objects become blurry to them.
At night, cats can only see if a minimal light source is available. Therefore, in pitch darkness, these animals rely on their other senses.
Cats can also see in the ultraviolet spectrum. If your pet is staring “into space,” perhaps he or she is watching for small particles of dust, an insect, or unusual-looking objects.
Hearing and smell. The cat may hear an unusual sound and therefore suddenly freeze in place. When it comes to smells, cats’ sense of smell is 14-16 times better than humans. Therefore, these pets also rely on their sense of smell when they notice an unfamiliar object.
episodic memory. It has been scientifically proven that cats can remember some moments in the past and dream. If a cat is staring into space, it may actually be remembering something at that moment.
Illness. Constant “clinging” of a pet to one place may indicate infectious diseases or disorders in brain function.
Source: Ferra

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