I’m really tired of seeing “Marr’s three levels” referenced.

The very first sentence of the abstract from Marr and Poggio (1976) is:

The CNS needs to be understood at four nearly independent level of description: (1) that a twhich the nature of a computation is expressed; (2) that at which the algorithms that implement a computation are characterized; (3) that at which an algorithm is committed to particular mechanisms; and (4) that at which the mechanisms are realized in hardware.[¹]

So, it’s four levels, not three, and it’s Marr and Poggio. Yes, I understand that it’s common today to collapse levels (2) and (3) together into one level, but that’s not what Marr and Poggio wrote. They clearly viewed them as distinct levels.

This is why it’s so important to actually read the original articles and not just keep doing indirect quotations.