Extremes touch each other. Since the Cretan Epimenides said: 'All Cretans are liars,' philosophers have built a discipline out of such paradoxes. In trying to resolve this particular one Bertrand Russell said he was reduced to 'wandering the common at night and staring at a blank sheet of paper by day'. I don't know what he concluded but Samuel Butler decided one one thing was certain, which was that nothing is certain. Including that it is not certain, that nothing is certain. When I was in my thirties I knew so much as to be sure of nothing anymore and could hardly express an opinion of any sort for a decade. The same ambiguities apply to the visual paradox. Now you see it, now you don't. Possibilities are shown to be impossible, and impossibilities probable. Although some of us find conundrums exceedingly irritating, this could be because they pose an unwelcome challenge to our perceptual apparatus - they are not unimportant. They remind us forcibly that things are not necessarily what they seem. "Art is a lie,' Picasso slyly explained, 'that makes us realise the truth.'
The grey line in the image above is horizontal, by the way.