Dear Matthew,
I would like to ask a few questions to understand you better:
MW:
Someone mentioned that an individual is something that does not have
members (in the sense of a set having members) and that is close to what
I would mean, but that would make the null set an individual, so it is
not quite adequate. My definition of individual is something that exists
in space and time. I am not a set (or class or type or kind or sort
etc) nor is my car or this email. Nor is Sherlock Holmes to give a more
difficult example. All these things can be placed in space and time
(even if it is imaginary space and time) . On the other hand,
sets/classes/types are generally considered to exist outside space and
time.
I understand that a 'main dichotomy' between an individual and a
set/class/type is - existence in space and time (rather than having or
not having members).
This depends on the metaphysics and language you've adopted. As Pat said, Individual means 'member of set' in logic and has nothing to do with space and time. Matthew's comments are couched in 4-dimensionalism so Individual is "Thing that exists in some possible world" and so is entirely about space and time. In 4-D, a Class can be a member of a Class, yet Classes are not Individuals.
So, without couching discussions into the approach/background that is the context, even Individual is not a clearly defined term.
MW: On the other hand,
sets/classes/types are generally considered to exist outside space and
time.
Can we consider opposite - sets/classes/types do exist within space and
time as well? For example, as a collection/set of space and time
extensions of all their members?
What will happen if the criteria is joined: 'does not have members' and
'exists in space and time'? Not sure if it is better... We have now
another (at least three) cases to think about…
As Classes are things of which Individuals are members based on some criteria so, by definition, Classes in 4D are not in space time. Classes are identified by their extension, but that does not put them into space/time either.
Cheers,
David