Should one use "the mechanics of the mind are" or "the mechanics of the mind is?
- The term "mechanics", used in this sense, seems to be an uncountable noun.
The meaning of the mechanics is something like
The sum of [the rules governing] the behaviour of the inner workings of
which, as you say, isn’t exactly countable but it’s a plural concept — each internal behaviour is a mechanic. Other ways to say the same thing are, e.g., inner workings or underlying rules, or …
Depending a little on what you’re trying to articulate you may find substituting mechanisms makes it sound better (and doing so as a “thought exercise” might help make the plurality more obvious).
Other examples that might help are: metabolism (singular) versus metabolic pathways (plural); skeleton versus bones; wood versus trees.
The actual answer to your question is left as an exercise to the reader.
[There's another example: exercises versus exercise régime]