One of my very favourite perceptual illusions. Quite famous, so you may well have seen it. But it still deserves a minute to contemplate in these times. /1

Square A and B are exactly the same colour and tone. Exactly. Seriously. Cut them out from the picture and compare them. /2

Why do we see A and B so differently? The clue is in the shadow thrown by the green block. Our brains see the shadow, and automatically adjust for the tone difference of B based upon the other information in the image. /3

But this illusion has a cooler philosophical element. Because it shows that what we think we 'see' - what we observe in our reality - isn't actually what 'is'. It's constructed from the image in our eyes, by our brains, to make a coherent reality. /4

Our perception is value-laden - subject to hidden adjustments and biases. So 'truth' or 'fact' - a word commonly bandied around to validate authority - isn't such a simple thing. The real 'truth' is, we can't completely untangle the human from the perceptive reality observed. /5

Not a bad revelation from a simple checkerboard and an old green block.... ;-) //

PS - this illusion also demonstrates the limits of what the politicians praise - and science condemns - as 'common sense'. Human assumption passing as universal truth. As prone to bias and flaw as any other prejudice.