If you didn’t already know this, it is interesting. Humans have an inherent knowledge of numbers,

typically up to about four. Try this with a bunch of pennies. Have a friend drop a few pennies in front of you. If there are 4 or fewer, you recognize the count instantly. It doesn’t matter how they are arranged (assuming all are visible), you *just know*. At six or seven there’s a pause. You find that you first have to pick out a 3 and a 4 and add them. You’re now calculating or counting. That’s cheating. It was not natural knowledge.

Also interesting is that the Japanese Soroban, abacus has 4 lower beads for units and a single upper bead for a five. If you have 4 set on the bar, to add one you zero the 4 and bring down the 5 bead. It’s a mini carry. The point being you see the 4 bead count instantly without cognitive load.

Animals too have this sense, although more limited. Most can tell the difference between one and two. There are human cultures where there are no names for numbers above the Subitizing range.

And perhaps this is why we are most comfortable manipulating numbers below five. This is the idea behind compliment arithmetic which converts adding a 7 to subtracting 3 and adding a ten. And that feels easier to most because 3 is within your Subitizing Power

If mental calculators had designed our number system, they might have been wise to choose a base 5 system. More verbose, true but easier to calculate with. If you think about it, this is what the Soroban is doing.