Limiting the unusual
When we talk about ‘all other things being equal’ we often mean no more
than ‘under normal conditions’. That is to say, we take it that there are no
unusual circumstances in the situation we are describing that might affect
the reasoning. For example, if we are discussing mass-murder ceteris paribus,
it is assumed that the murderer was not given an ultimatum stating
that if he did not kill 20 people by noon the whole world would be blown
up. But the phrase ‘under normal conditions’ does not capture the full scope
of ceteris paribus, which, as we have seen, can be invoked in thought experiments
where conditions are, by definition, not normal.