The mental body — called the mental body in Theosophical tradition and corresponding to the vijnanamaya kosha (wisdom sheath) and manomaya kosha (mind sheath) in Vedantic philosophy — is the third layer of the human constitution, interpenetrating and extending beyond the physical and astral bodies. It is the layer of thought, concept, language, belief, meaning and the narrative structures through which experience is organised and interpreted.
Where the emotional body responds to experience directly and immediately — feeling arises before thinking, always — the mental body processes what the emotional body has already registered and gives it meaning. The same event can produce entirely different experiences depending on what the mental body does with it. Two people lose their jobs: one interprets it as catastrophic failure and spirals into depression; the other interprets it as an unexpected opportunity and becomes energised. Same event, different mental bodies, entirely different experiences. The mental body is the meaning-making layer — and meaning shapes experience more powerfully than the raw events that trigger it.
In Theosophical cosmology the mental body operates on two sub-planes: the lower mental plane (concrete mind — the linear, analytical, sequential thinking that solves problems, plans and reasons) and the higher mental plane (abstract mind — the intuitive, pattern-recognising, synthesis-making capacity that grasps wholes rather than parts). These correspond roughly to what contemporary neuroscience calls left-hemisphere processing (sequential, analytical) and right-hemisphere processing (holistic, pattern-recognising), though the Theosophical map is considerably more detailed. Most people have well-developed lower mental function and almost entirely undeveloped higher mental capacity — the capacity to think in principles, archetypes and living wholes rather than in linear sequences of cause and effect.