<P> The paradox appears in various forms in fictional contexts, particularly in fantasy or science - fiction, for example where a character has body parts swapped for artificial replacements until the person has been entirely replaced . There are many other variations with reference to the same concept in popular culture for example axes and brooms . </P> <P> The Greek philosopher Heraclitus attempted to solve the paradox by introducing the idea of a river where water replenishes it . Arius Didymus quoted him as saying "upon those who step into the same rivers, different and again different waters flow". Plutarch disputed Heraclitus' claim about stepping twice into the same river, citing that it cannot be done because "it scatters and again comes together, and approaches and recedes". </P> <P> According to the philosophical system of Aristotle and his followers, four causes or reasons describe a thing; these causes can be analyzed to get to a solution to the paradox . The formal cause or' form' (perhaps best parsed as the cause of an object's form or of its having that form) is the design of a thing, while the material cause is the matter of which the thing is made . Another of Aristotle's causes is the' end' or final cause, which is the intended purpose of a thing . The ship of Theseus would have the same ends, those being, mythically, transporting Theseus, and politically, convincing the Athenians that Theseus was once a living person, though its material cause would change with time . The efficient cause is how and by whom a thing is made, for example, how artisans fabricate and assemble something; in the case of the ship of Theseus, the workers who built the ship in the first place could have used the same tools and techniques to replace the planks in the ship . </P> <P> According to Aristotle, the "what - it - is" of a thing is its formal cause, so the ship of Theseus is the' same' ship, because the formal cause, or design, does not change, even though the matter used to construct it may vary with time . In the same manner, for Heraclitus's paradox, a river has the same formal cause, although the material cause (the particular water in it) changes with time, and likewise for the person who steps in the river . </P>

The part of the experiment that does not change