When I studied logic at University, I learned:
if p, then not not-p;
not (p and not-p);
either p or not-p.
example, if it is the case that Socrates was executed in 399 BC, then
it is not the case that Socrates was not executed in 399 BC - unless
there are alternative timelines, in which case logical consistency is
maintained by making our propositions, p and not-p, more
specific. Thus, if it is the case that Socrates was executed in 399 BC
in timeline 1, then it is not the case that Socrates was not executed in
399 BC in timeline 1.
This may seem obvious but I meet
people who get their idea of logic not from Aristotle or his successors
but from Mr Spock. "Logic" means something like thinking rigidly and
unemotionally instead of just thinking and speaking consistently which
everyone tries to do. No one openly contradicts himself on a matter of
fact, then says, "I am free to contradict myself because I am not bound
by logic like Mr Spock." And anyone who did say that would not succeed
in telling us anything. "Socrates both was and was not executed in 399
BC in timeline 1 and I am free to contradict myself..."
might be imagined that a rigidly "logical" thinker, having denied that
Socrates could both be executed and not be executed in 399 BC, would
then compound his rigidity by denying that there can be alternative
timelines. Merely to reply that Mr Spock experiences alternative
timelines is to confuse a conceptual question with an empirical
A conceptual question: Are alternative timelines possible?
Answer: Yes. There is no reason why not. No contradiction is involved.
An empirical question: Do alternative timelines in fact exist?
Answer, within the framework of the Star Trek narrative: Yes. They have been discovered and entered.
Not only Star Trek. We are grateful for the alternative timelines of Poul Anderson, SM Stirling and Harry Turtledove.