That sure looks like insect damage to me ... the mint won't do much to the basil (except try to take over the world!). From the pattern, I'd guess four lined plant bugs or maybe a leaf miner but you'd have to go out and check the leaves to see what critters are running around. If it's four lined plant bugs, then the basil should just outgrow the damage.

Slugs or Japanese Beetles would eat more of the leaves.

Thanks. I'll try removing the affected leaves and see what happens.

Makalu, Thasnk you again.

I checked my plants and you are correct - I saw some four lined bugs there. I cut away some of the damaged leaves (along with the bugs on them).
From what I've read, the problematic time from these bugs is limited, so I hope it'll resolve itself.