[Note: I have posted something very similar to this somewhere on MBTICentral before; this is a rewritten version.]
Suppose that a scientist is searching for theories which correspond to the facts, and by convention he labels such theories 'true.' One day a philosopher asks the scientist whether he has succeeded. Although the scientist has discovered some useful theories, each has its problems and he does not think any are true. But the scientist is then told by the philosopher that his theories must be true. He is then convinced by the philosopher that a theory is not true because it corresponds to the facts, but because it has instrumental value. Disabused of the correspondence theory of truth, the scientist now adopts to the pragmatic theory of truth. Prior to his conversion, the scientistís search for truth meant searching for theories which correspond to the facts, but now that is replaced by a search for theories with instrumental value.
But is this a sensible move by the scientist? The original aim of the scientist's search was the discovery of theories which correspond to the facts, and even though he may no longer label such theories 'true,' nothing else need change.
Suppose that the scientist lost his car keys and searched thoroughly to no avail. The philosopher then convinces him that the word 'key' really refers to a writing utensil, and therefore, advises that he search for that instead. Now disabused of the locking theory of key, the scientist adopts the writing theory of key, and begins searching for a writing utensil. Shortly afterward the scientist declares that he has found his key, and triumphantly uses it to write a letter to his car's manufacturer asking how to turn the engine on.
Words do not have real or essential meanings. For example, the word 'post' can refer to many different things--including a piece of wood set upright into the ground as a marker, a starting point at a racetrack, or an electronic message sent to a forum. But do these alternative interpretations constitute theories of post? Would it be sensible to argue about which theory of post is correct? The meaning a word is not a matter of discovering its real or essential meaning, but of negotiating a conventional use and interpretation. And although this principle is uncontroversial regarding words like 'key' and 'post,' it is often forgotten when discussing the word 'truth.'