ronan said:
So I believe in this reality behind our perceptions and I understand why Planck use the words direct perception instead of belief even if I would not use these words.
I too think I understand why Planck used such language, and I regard it as a fundamental metaphysical error.
I think he approaches reality from the outside - as if his mind were outside reality, attempting to perceive it more or less as one perceives an object by seeing it.
This approach, reflected in his use of "direct perception", contains something I regard as a deep insight - that mathematics and scientific reasoning operate as a sense organ for perception of matters and properties and features unavailable to our physical sensory apparatus. If we were totally blind, we would "see" by such endeavor alone, for example. One might imagine a fishlike midocean intelligence having no approach to geometry other than mathematical.
But the serious misapprehension it leads to is also important - the reality is not "behind" our perceptions as the tree is "behind" the appearance of the bark and leaf, but as our eyeball is "behind" our seeing of that bark and leaf. One doesn't directly perceive the existence of reality - one perceives, directly and otherwise, as a function of that reality.
So the faith necessary is not in a correspondence of reality and perception brought about by the a priori or Creator established reliability of the perception in reflecting reality, but in the assumed or deduced common origin of the perceiving and the perceived. Common source, common nature, in some ways at least.