the extended intelligence theory

Try to develop a complete and correct algorithm for a complicated programming problem in your head. Try to do a complete mathematical proof in your head for an important theorem you haven't proved before (in a branch of mathematics whose axioms you understand well). Try to seriously think through complicated philosophical territory in the philosophy of language in your head.

Try to solve any major intellectual problem in your head.  Just in your head, with no paper at all. It shouldn't be so challenging...

It's impossible. Working memory gets overloaded so quickly.

Not sure to what extent people realise, as F. Chollet does (https://medium.com/@francois.chollet/the-impossibility-of-intelligence-explosion-5be4a9eda6ec), that it is technology and culture that has raised us far above our fellow organisms cognitively, not the cognitive endowment itself (except insofar as it allowed for the possibility of complex culture to begin with). Racist people from history or today who mock and disdain the primitive technology of hunter-gatherers (a group that includes great philosophers like Hume and Kant) fail to realise that, if a person with their genetic recipe were raised in such an environment (if 'they' were raised in that environment), they would just  be yet another hunter-gatherer armed with a bow and arrow, bawdy jokes, a fanatical attachment to bizarre and traumatic initiation rituals, and a very rich botanical and ethological knowledge of their local environment (i.e. they wouldn't fucking invent a slingshot, let alone writing, let alone prove one of Euclid's theorems, let alone...). People do not sufficiently appreciate the fact that the 20th and 21st Century intellectual giants we recognise in the natural and mathematical sciences - Einstein, Curie, Turing, von Neumann etc - who made astonishing advances unprecedented in history, did not have a kind of brain-wiring or processing speed unprecedented in humans in history. This should be obvious, but I get the impression many people don't reflect on it deeply. Our intelligence as humans is so culture-dependent, even if this doesn't show up so much on IQ tests (except it does a bit in any case, says the Flynn Effect).