Describe the basic concept behind criminal responsibility regarding infancy and diminished capacity?

The basic concept here is that a person cannot be found
guilty of a crime if the person was, for whatever reason, incapable of understanding
that the act they were committing was illegal.  Such a person would not have the
appropriate mental state to be found guilty of a
crime.

Legally speaking, there is the idea that a person
must reasonably be able to know their conduct was illegal.  They must be able to
understand the nature of their act (that it is illegal) and its consequences (that it
can result in them being punished).  People who are too young or who are otherwise
impaired (such as by mental illness) are deemed to be incapable of understanding these
things.

This points to a basic concept -- that an action is
only criminal if the person committing it is capable of understanding that it was
criminal.  It is not only the act that makes the crime, it is the act plus the mental
state of the actor.