I wonder who first drew a diagram?
According to one definition, “diagrams are simplified figures, caricatures in a way, intended to convey essential meaning”*.
That seems about right to me.
Some diagrams are so good at this, that once seen, you can’t help but assimilate the essential meaning. In an instant, it’s there, in your head forever, changing how you think from that point onwards.
This is one of my favourites:

My interpretation of a diagram from Alan Begg and Graham Williams
The explanation goes something like this. We all operate best in ‘Can Do’ mode, creative, autonomous, responsible, positive, active. But when knocked back for whatever reason, we have a tendency to slip down into one or other of the legs of the diagram.
If we go down ‘Can’t Do’, we become helpless, we freeze up, we become inactive and cautious. If we go down the ‘Won’t Do’ leg, we blame others, we feel resentful, angry, we become unco-operative, even disruptive.
The interesting thing is that all three behaviours have upsides. There are advantages to being in ‘Can’t Do’ or ‘Won’t Do’ that we may learn to exploit, and so keep ourselves there, instead of learning how to get ourselves back to ‘Can Do’, where we operate at our best.
But the key point is that these behaviours are learned. Which means we can unlearn the restricting ones, and learn to get back into ‘Can Do’ mode more quickly and easily, to the benefit of ourselves and the people around us.
I know where I go – I’m straight off down the ‘Can’t Do’ leg – but I also know how to get myself back up again quickly – and all I need to remind me how to do that is a glance at this simple diagram.
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* Bert S. Hall (1996). “The Didactic and the Elegant: Some Thoughts on Scientific and Technological Illustrations in the Middle Ages and Renaissance”. in: B. Braigie (ed.) Picturing knowledge: historical and philosophical problems concerning the use of art in science. Toronto: University of Toronto Press. p.9