Philosophy requires stretches of the imagination in a way that science fiction prepares us for. It is for this reason that I think the latter can often serve as a useful and entertaining supplement when investigating the former. Examples of the close connection between these two areas of thought abound. But I’ll make the point with just two.

Philosophers have spilled a large amount of ink over the question of personal identity. Sci-fi authors and screenwriters have done the same. The episode “Second Chances” from Star Trek: The Next Generation begins with the discovery of a duplicate of Commander Riker on a planet he visited eight years before. The Enterprise crew determines that a transporter accident resulted in the materialization of two Rikers—one who made it back to the ship and the other who rematerialized on the planet. Which of these two Rikers is identical to the one that existed before the accident? This episode explores this question in detail. Indeed, such cases have become commonplace in the literature on personal identity.

Another example: For many years, philosophers thought that to know the meaning of a word was just to be in a certain psychological state—that meaning is wholly in the head. In his 1973 paper ‘Meaning and Reference’, Hilary Putnam set out to disprove this. To do so, he asked us to suppose there was a planet very similar to the earth, which he called ‘Twin Earth’, inhabited by beings similar to us. Each of us has a duplicate on Twin Earth. The only difference between earth and Twin Earth is that, wherever there is H2O on earth, there is a different substance, which Putnam called ‘XYZ’, on Twin Earth. On the macroscopic scale, this substance looks

and behaves just like H2O. It’s only upon inspection at the microscopic scale—at the atomic level—that the differences between the two substances become apparent.

Putnam then asked us to consider someone, Oscar, on earth in 1700, before the discovery of the microstructure of water, who uses the word ‘water’, and consider his duplicate on Twin Earth, before the discovery of XYZ, who does the same thing. Do Oscar and his duplicate mean the same thing by ‘water’? The conception Oscar and his duplicate associate with ‘water’ are the same—things like clearness, thirst quenching, flowing in streams and rivers, abode of fish. They are in the same psychological state. But Putnam argues that this cannot be the case. Oscar, by using ‘water’, means one substance (H2O—that’s the stuff he’s talking about), while his duplicate, using the same word, means an entirely different substance (XYZ—that’s what he’s talking about).

Four years before Putnam’s paper came out, the film Journey to the Far Side of the Sun (known as Doppelgänger in Europe) was released. Its premise is that a duplicate of the earth follows the same orbital path around the sun, but is at all times situated on the exact opposite side of the sun relative to earth. A mission is sent to this newly discovered planet. While Journey lacks the philosophical grist that characterizes Putnam’s paper, one has to wonder whether Putnam had seen the movie, and that it had gotten him thinking. Similarly, one can’t help but wonder what state the debate on personal identity would be in if the developers of Star Trek had not come up with the idea of the transporter as simply a way to save time and money.