My dad cured me of that by pointing out that if I had that third arm there would always be a project where I'd want a fourth anyway. It's the same argument that Danny Hillis gave me for the 1-bit ALU in the original connection machine design: "well, what number is correct? This way you can build any size you want" which was almost true.

However the second thumb is a great idea. I have long though that if there were some sort of god it must have been quite mean to choose five as the number of digits. Six, with an extra opposable thumb on the opposite side, would have been much much better (and even the same architecture but with a total of four would have been superior to the current setup).

Also annoying was turning the knobs for pi and e to irrational values.

Just because there are scenarios where you'd want a fourth doesn't invalidate the desire to want a third. There are more situations where a third arm would be helpful than where a fourth would be. The more arms, the fewer situations apply.

Well two is a good optimization: it gives you a portable opposition point (with one arm you spend a lot of time pushing something against a wall or something like that -- try living with your arm in a cast) vs needing enough musculature to make the arms worth operating yet not make it heavy.

Plus there's a path dependency issue (the world is set up with chair backs, shirts, whatever assuming bilateral symmetry). But evolutionarily (reimagining the evolution of humans rather than adding an auxiliary arm) the path dependency is not meaningful.

That argument only makes sense if your utility function were either monotonic and non-decreasing over arms or if it were monotonic and non-decreasing over a combination of arms and other things (e.g. legs?). Are you sure you'd always prefer having more arms?

I mean, I don't know about you, but I think most people would stop short of becoming a hecatoncheir. At best those extra arms would be useless, but in the worst case scenario you could be punished by the gods for your hubris. Either that, or be banished to Tartarus by Zeus.

Look at a parrot's foot some time, and let the envy over the gripping ability of their zygodactyl toes flow through you.

Perfect for soldering.