I think Kant would disagree here, and I’m inclined to take his side on this:

1. All phenomena are presented to us through the formal intuitions of space, time, and causality
2. These categories are anthropocentric: they structure human experience
3. There is a world outside of human experience
4. Therefore, there is a world outside of space, time, and causality, which exists but which cannot be presented to us phenomenally

So I think the case could be made that it *is* an irrational position to deny the existence of anything outside the phenomenal realm.