A physical eye, similar to a mirror, may reflect an external image/object in it. However, the mental property/element of eye consciousness is require for the mind to experience/know the external image/object/sight.
Similar to the physical eye, the mano-dhatu is the sense organ required for the mind to know things/objects that can only be sensed/experienced/known by mano-dhatu. The things/objects that can only be sensed by mano-dhatu include:
- Thoughts and defilements (moods, emotions, drives)
- Consciousness itself
- Truth, such as Four Noble Truths, impermanence, unsatisfactoriness, not-self, emptiness
While the Pali suttas do not explicitly define 'dharma-dhatu', obviously 'dharma-dhatu' are far more extensive than mere 'mental objects' (feelings, perceptions & thoughts) because they must include Truth (Insight) and Nibbana. Obviously, this is why the word 'dhamma' is used (rather than a term that is exclusively mental, such as "cetasika" or "nama-dhamma").
Originally, Bhikkhu Bodhi properly translated dhamma-dhatu (in Pali) as "mind-objects"; meaning "objects known by the mind". But recently, he has changed his translation to "mental phenomena", which is wrong (because Truth and Nibbana are not mental phenomena or nama-dhamma). The translations of Thanissaro and Sujato Bhikkhu of "ideas" and "thoughts" is also wrong.