Chapter 16: “Who Did You Pass On The Road? Nobody”: Lojban And Logic
4. Restricted claims: “da poi”
The universal claims of c16-§3 are not only false but absurd: there is really very little to be said that is both true and non-trivial about every object whatsoever. Furthermore, we have been glossing over the distinction between “everything” and “everybody” and the other pairs ending in “-thing” and “-body”. It is time to bring up the most useful feature of Lojban variables: the ability to restrict their ranges.
In Lojban, a variable “da”, “de”, or “di” may be followed by a “poi” relative clause in order to restrict the range of things that the variable describes. Relative clauses are described in detail in Chapter 8, but the kind we will need at present consist of “poi” followed by a bridi (often just a selbri) terminated with “ku'o” or “vau” (which can usually be elided). Consider the difference between
✥4.1 da zo'u da viska la djim. There-is-an-X : X sees Jim. Something sees Jim.
✥4.2 da poi prenu zo'u da viska la djim. There-is-an-X which is-a-person : X sees Jim. Someone sees Jim.
In ✥4.1, the variable “da” can refer to any object whatever; there are no restrictions on it. In ✥4.2, “da” is restricted by the “poi prenu” relative clause to persons only, and so “da poi prenu” translates as “someone.” (The difference between “someone” and “somebody” is a matter of English style, with no real counterpart in Lojban.) If ✥4.2 is true, then ✥4.1 must be true, but not necessarily vice versa.
Universal claims benefit even more from the existence of relative clauses. Consider
✥4.3 ro da zo'u da vasxu For-every X : X breathes Everything breathes
✥4.4 ro da poi gerku zo'u da vasxu For-every X which is-a-dog : X breathes. Every dog breathes. Each dog breathes. All dogs breathe.
✥4.3 is a silly falsehood, but ✥4.4 is an important truth (at least if applied in a timeless or potential sense: see Chapter 10). Note the various colloquial translations “every dog”, “each dog”, and “all dogs”. They all come to the same thing in Lojban, since what is true of every dog is true of all dogs. “All dogs” is treated as an English plural and the others as singular, but Lojban makes no distinction.
If we make an existential claim about dogs rather than a universal one, we get:
✥4.5 da poi gerku zo'u da vasxu There-is-an-X which is-a-dog : X breathes. Some dog breathes.