[beavus]

But you can't use a hammer to explain a hammer, to understand it, or to define it

You can't use an actual hammer to explain anything, because that isn't what hammer do. Hammers pound things. Reason explains things.

You can however use an actual hammer to understand hammers (look at it, pound with it, generally interact with it), and you can use an actual hammer to define hammers (which ultimately, with some abstraction across many different observations of hammers, is what we do).

You cannot, however, use the notion hammer as an explanation of what a hammer is. Instead, you'd have to introspect, peruse your memories, and find out what observations or non-hammer concepts you used to form the concept of hammer.

[HangnJudge]

You cannot, however, use the notion hammer as an explanation of what a hammer is. Instead, you'd have to introspect, peruse your memories, and find out what observations or non-hammer concepts you used to form the concept of hammer.

Ah, but now you are using reason to study the hammer, not the hammer itself
We still have the problem of using Reason to explain Reason,
Logic to understand Logic