4 ideas
Renouvier's 1876 alternate history branches at the end of Marcus Aurelius's reign, when the imperial succession is secured by capable men rather than passing to Commodus. The West then contains Christianity and other Eastern salvation cults, pushing them toward the East. Europe develops along Roman-philosophical lines toward civic freedom and tolerance, and Christianity survives as one minor creed among many rather than a state church.
A uchronia ('no-time') applies the logic of utopia ('no-place') to history. It describes not an ideal society somewhere else but the course events could have taken if different choices had been made at real turning points. The imagined society is anchored to an actual historical branch point instead of floating free as a fantasy.
Because humans have real free will, the alternatives open at any decisive moment were genuinely possible, not merely imagined. Determinist philosophies of history work backward from what happened and relabel it as what had to happen, erasing the other options. Showing a coherent alternative course demonstrates that the actual outcome was contingent on choices.
If an outcome is treated as inevitable, the people who produced it are excused, since no one is blamed for necessity. Laying out a plausible better path turns each historical event back into a decision that could have gone otherwise. That decision can then be judged, and the lesson carries forward: present choices are equally undetermined and equally accountable.