Why but because of

Hello,

The underlined part is difficult.
What does it mean?

And what the bold typed "this" refers to is also difficult.
What does the "this" refer to?

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W 151 L 2. You do not seem to doubt the world you see. You do not really question what is shown you through the body's eyes. Nor do you ask why you believe it, even though you learned a long while since your senses do deceive. That you believe them to the last detail which they report is even stranger when you pause to recollect how frequently they have been faulty witnesses indeed! Why would you trust them so implicitly? Why but because of underlying doubt which you would hide with show of certainty?

W 151 L 3. How can you judge? Your judgment rests upon the witness that your senses offer you. Yet witness never falser was than this. But how else do you judge the world you see? You place pathetic faith in what your eyes and ears report. You think your fingers touch reality and close upon the truth. This is awareness which you understand, and think more real than what is witnessed to by the eternal Voice of God Himself.