The top one, I answered.
Of course it does, said Zafar. Now take this other napkin and line it up to check you’re right. Humor me.
I should have should have seen it coming. As I brought the edge of the folded napkin against the diagonal in the top left of the diagram, it became apparent that this line extended downward not to the top diagonal, as I had said, but to the bottom one.
This is Poggendorff’s illusion, said Zafar. Johann Poggendorff, he continued, was a nineteenth-century German physicist and the creator of a number of measuring devices. Your father will probably have heard of him. There are countless optical illusion of a similar type---you probably know the Müller-Lyer illusion: two parallel lines with arrows at the end, arrows inverted at one of the lines; which line is longer?
I know that one, I said.
But it’s Poggendorff’s illusion I like the most, because it reminds me of the distinction between a reason for doing something and an incidental benefit of doing it. But I’ll come to that. You say that once we know how the world actually is---once we see it correctly---we can fix things. Now that you know what the truth here is, let me ask you one more time: Which of these two diagonals on the right, the top one or the boom, which of them looks---and I mean looks---like it’s the extension of the diagonal on the left?
The same. Nothing’s changed, I replied. It looks the same as before.
Knowing how things are doesn’t make you see them correctly, doesn’t stop you from seeing things incorrectly. Stare at the image as much as you like, it’s all in vain. It will never surrender the truth, not to your naked eyes; you have to go in armed with a straightedge.
---from In The Light of What We Know by Zia Haider Rahman.