Where am I assuming that we have perfect knowledge of physics?

To make it plain, I'll break the argument in two parts:

(a) if AGI is impossible but humans are intelligent, then it must be the case that human behavior can't be explained algorithmically (that last part is Penrose's position).

(b) the statement that human behavior can't be explained algorithmically is about physics, not mathematics.

I hope it's clear that neither (a) or (b) require perfect knowledge of physics, but just in case:

(a) is true by reductio ad absurdum: if human behavior can be explained algorithmically, then an algorithm must be able to simulate it, and so AGI is possible.

(b) is true because humans exist in nature, and physics (not mathematics) is the science that deals with nature.

So where is the assumption that we have perfect knowledge of physics?

That's true, but we should acknowledge that this question is generally regarded as unsettled.

If you accept the conclusion that AGI (as defined in the paper, that is, "solving [...] problems at a level of quality that is at least equivalent to the respective human capabilities") is impossible but human intelligence is possible, then you must accept that the question is settled in favor of Penrose. That's obviously beyond the realm of mathematics.

In other words, the paper can only mathematically prove that AGI is impossible under some assumptions about physics that have nothing to do with mathematics.

I remember that one: the two blacks were not slightly different, they were both exactly black but written in different ways.

The image was in PPM format, which stores the color components of the pixels as ASCII text (so a white pixel is stored as "255 255 255" and a black one is "0 0 0"). To redact the image, the code replaced every digit of the numbers with '0', so white became "000 000 000" and black stayed as "0 0 0". Both are black and indistinguishable if you're viewing the image, but you can tell them apart by looking at the file text.

Sadly the UCC homepage seems to have vanished, but I found this account from the author: http://notanumber.net/archives/54/underhanded-c-the-leaky-re...

That's not it, the lower case 'a' is very different.

Not sure if you're being snarky, but the manual has a list of all options accepted by make: https://www.gnu.org/software/make/manual/html_node/Options-S...

(`make --help` will only print the most common options)