Maybe you misunderstood what that bhikkhu was saying (or did he actually mean it)? The following is my understanding.
1. Yes. The world that I experience is mental and “real” for me.
2. But my mental experience is based on sensory inputs coming from the external world, and that external world is also real.
3. However, another person may experience the same external world differently from me.
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I think the above points describe the fundamental basis.
- In other words, everyone experiences the same external world.
- However, the “mental experience” depends on the person. The simplest example is the following. Two people, A and B, look at person X (who is a friend of A and an enemy of B). A experiences joy upon seeing X, but anger arises in B upon seeing the same person X.