Counterfactuals are interesting to philosophers because they ask us to think about what would be true if something had been different. They live in the world of “what if.” 

What if kangaroos had no tails, would they topple over?

Which you can turn into a statement like this:

If kangaroos had no tails, they would topple over.

All this talk about kangaroos became famous with the American philosopher David Lewis, who visited Australia frequently. He famously handled counterfactuals using possible worlds. In his view, you would evaluate those statements by looking at the closest possible worlds where kangaroos have no tails. If, in those nearby worlds, kangaroos topple over, then the counterfactual is true. Lewis’s approach asks us to temporarily grant an imagined premise.

The Idea of Ideas handles counterfactuals more simply. It treats them like any other idea. The word “if” does not give an idea special permission to pretend to be true. Furthermore, counterfactuals are rational or irrational ideas. They are never empirical because they never describe the material world directly.

Here is an example of a rational counterfactual:

If the room had no oxygen, the fire would not have started.

This is rational because it describes a possible world that rests on a directly testable and repeatable relationship in the material world. Fire requires oxygen.

Now, take this rational idea:

If something that is one inch long were two inches long, it would be twice as long.

This is a counterfactual because it reimagines one inch things.

As one final example, let’s take a look at a speculative counterfactual.

In a world where kangaroos had no tails, they would topple over.

This is speculative because the claim depends on an imagined world that is not known to exist but also cannot be disproven.